Parallel evolution in human populations: A biocultural perspective

Abstract

Parallel evolution—where different populations evolve similar traits in response to similar environments—has been a topic of growing interest to biologists and biological anthropologists for decades. Parallel evolution occurs in human populations thanks to myriad biological and cultural mechanisms that permit humans to survive and thrive in diverse environments worldwide. Because humans shape and are shaped by their environments, biocultural approaches that emphasize the interconnections between biology and culture are key to understanding parallel evolution in human populations as well as the nuances of human biological variation and adaptation. In this review, we discuss how biocultural theory has been and can be applied to studies of parallel evolution and adaptation more broadly. We illustrate this through four examples of parallel evolution in humans: malaria resistance, lactase persistence, cold tolerance, and high‐altitude adaptation.

Full text

Received: 24 September 2021
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Revised: 9 March 2022
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Accepted: 30 July 2022
DOI: 10.1002/evan.21956
REVIEW ARTICLE
Parallel evolution in human populations: A biocultural
perspective
Christina M. Balentine
1,2
|Deborah A. Bolnick
2,3
1
Department of Integrative Biology,
University of Texas at Austin, Austin,
Texas, USA
2
Department of Anthropology, University of
Connecticut, Storrs, Connecticut, USA
3
Institute for Systems Genomics, University of
Connecticut, Storrs, Connecticut, USA
Correspondence
Christina M. Balentine, Department of
Integrative Biology, University of Texas at
Austin, Austin, TX, USA.
Email: cmbalentine@utexas.edu
Abstract
Parallel evolution—where different populations evolve similar traits in response to
similar environments—has been a topic of growing interest to biologists and
biological anthropologists for decades. Parallel evolution occurs in human popula-
tions thanks to myriad biological and cultural mechanisms that permit humans to
survive and thrive in diverse environments worldwide. Because humans shape and
are shaped by their environments, biocultural approaches that emphasize the
interconnections between biology and culture are key to understanding parallel
evolution in human populations as well as the nuances of human biological variation
and adaptation. In this review, we discuss how biocultural theory has been and can
be applied to studies of parallel evolution and adaptation more broadly. We illustrate
this through four examples of parallel evolution in humans: malaria resistance,
lactase persistence, cold tolerance, and high‐altitude adaptation.
KEYWORDS
adaptation, biocultural theory, biological anthropology, biological sciences, human evolution,
parallel evolution, transdisciplinary research
1|INTRODUCTION
Parallel evolution—the process by which different populations evolve
similar traits in response to similar environments—is of keen interest to
biological anthropologists and biologists more broadly.
1–4
The study of
parallel evolution allows scientists to observe repeated instances of
evolution within the world itself, rather than in a controlled laboratory
environment, which can shed light on how the many random (e.g., genetic
drift) and nonrandom (e.g., adaptation) processes acting on populations
interact to shape biological diversity.
4
Parallel evolution has been studied
extensively in nonhuman organisms,
1,4
and within evolutionary anthro-
pology, decades of research have investigated convergent traits in fossil
hominoids,
5
shared physiology in extant primates,
6
and similarities among
stone tool technologies in groups inhabiting disparate parts of the globe.
7
However, such research in more recent ancient and present‐day human
populations has been more limited.
2
Nevertheless, thanks to myriad
biological and sociocultural mechanisms, humans survive and thrive in
nearly every environment of the world, leading to groups around the
globe with distinct demographic histories that sometimes adapt in similar
ways to similar environments
8–11
(Box 1).
Because humans are unique in the degree to which we rely on
culture to thrive in our habitats, it is imperative to consider both the
biological and cultural mechanisms contributing to parallel evolution—
and adaptation more broadly—within human populations. Biocultural
theory (Box 2) provides an especially useful framework for examining
the various mechanisms that contribute to parallel evolution. While
biocultural approaches have been used for decades to highlight the
interconnections between sociopolitical economy and health out-
comes in human populations,
25,29
genetic adaptation studies have less
frequently emphasized the interplay between biological changes and
cultural developments in human evolution. Specifically, studying
parallel evolution in humans from a biocultural perspective provides
valuable insights into human adaptability. Numerous instances of
parallel evolution have occurred in human groups across the globe.
8–11
Evolutionary Anthropology. 2022;1–15. wileyonlinelibrary.com/journal/evan © 2022 Wiley Periodicals LLC.
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1
[Correction added on 29 September 2022, after first online publication, reference list has been renumbered]

BOX 1 Glossary
Parallel evolution: the process of similar traits evolving in response to similar selective pressures. Following recent papers on the topic, we
make no distinction between parallel and convergent evolution, and parallel evolution can occur in organisms with similar or divergent
ancestries.
4,12
Adaptation: either (a) the process of natural selection acting on a phenotype or genotype to increase its frequency in a population over
multiple generations or (b) the resulting trait that is beneficial in a particular environment.
Population: (in the social sciences) a group of individuals who share similar socio‐politico‐cultural and biotic environments; (in population
genetics) any group of geographically proximate interbreeding individuals who can produce viable offspring.
Biocultural theory: a theoretical framework within anthropological research that has no precise definition, but rather denotes the
overarching goal of examining human biological variation while giving equal consideration to the biotic and sociocultural environments that
people inhabit, and highlights the interconnections between biology and culture.
Selection: an evolutionary process that shapes biological diversity via the differential survival and reproduction of individuals due to
differing phenotypes. Positive selection increases the frequency of a beneficial variant, while negative selection decreases the
frequency of a deleterious variant. Balancing selection maintains multiple variants at high frequencies (balanced polymorphisms), even
if one variant is deleterious when homozygous.
Plasticity: the biological capacity for an individual to change some aspect of their phenotype in response to environmental conditions.
Genotype: the genetic makeup of an individual; may be a pair of nucleotides/alleles at a locus in Mendelian genetics or the
combination of alleles in polygenic traits (traits influenced by multiple loci).
Phenotype: a visible and/or measurable trait of an individual that is the outcome of that individual's genotype, gene expression, and
environmental inputs.
Gene: a segment of DNA sequence that encodes a protein and is expressed phenotypically.
Locus (pl. loci): a location in the DNA; both genes that encode proteins and noncoding regions of the genome are considered loci.
Allele: a specific version or variant of a locus (i.e., the specific nucleotide sequence of the DNA at that location in the genome).
Haplotype: a stretch of a chromosome that is inherited as a unit.
Acclimation: a biological response to an environmental stressor that occurs within one's lifetime and is reversible.
Epigenetics: modifications to DNA that are not in the DNA sequence itself; includes molecules that are attached to specific nucleotides
in the DNA and proteins that influence the folding of the chromosome, which impacts the expression of genes.
BOX 2 Integrating biocultural theory with human adaptation research
Anthropological interest in human adaptation can be traced back to the very beginnings of our field.
13,14
However, it was not until the 1960s,
when the International Biological Programme's Human Adaptability Program (IBP‐HAP) began to systematically study human adaptation
through a biological lens, that anthropology entered “a‘golden age’for the study of human adaptability.”
13, p. 815
Despite this characterization,
the IBP‐HAP and other adaptation studies were repeatedly critiqued for (1) not incorporating social science perspectives,
13,15
(2) breaking
organisms down into parts to study separately rather than as a whole, and (3) assuming traits observed today are the result of natural selection
rather than other evolutionary processes.
16
Some of this study was also problematic because it was rooted in racist, sexist, classist, and white
supremacist ideologies.
Today, in the post‐human genome era of adaptation research, some of these criticisms are still valid.
17
Modern genetic adaptation studies
rely on mathematical algorithms to identify signals of natural selection in the genome, and some weave stories about the function and
significance of variants identified as putatively adaptive without much knowledge of the associated phenotypes, relevant sociocultural
contexts, or even whether selection or other evolutionary processes are actually responsible for the patterns of variation observed.
18
Furthermore, assuming a simple relationship between biology and the environment, where biotic environments shape genetic variation and
human biologies primarily through natural selection, is problematic. Humans are not passive acceptors of our environmental circumstances
19,20
;
we manipulate our environments and behavior to create more hospitable lives in even the harshest contexts.
21
Cultural developments and
sociopolitical environments may also influence adaptation, although they are not always considered,
22
and the phenotypes we see often reflect
factors other than—or in addition to—selection. Whether intentional or not, traditional approaches to adaptation research frequently lend
themselves to overly simplistic thinking and biological or genetic determinism. In some cases, these studies may also contribute to the
reification of racist ideologies and victim‐or ancestor‐blaming. For example, some adaptation studies have contributed to the misconception
that Indigenous groups in the Americas are maladapted to the modern world and that the high frequency of diabetes in Indigenous
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communities is the result of selection related to their ancestors’high‐fat animal‐based diets rather than the lack of healthy foods, healthcare,
and other resources available to Indigenous groups today.
23
Incorporating biocultural theory into adaptation research can help us develop more holistic, antiracist, and anti‐deterministic research
programs. Biocultural research has been at the fringes of physical/biological anthropology for over 60 years but has long been seen as an
alternative to some of the problematic adaptationist research in this field (for more comprehensive histories of biocultural anthropology, see
the 1998 volume Building a New Biocultural Synthesis,
24
Hoke & Schell,
25
and Wiley & Cullin
26
). It is now a key framework in medical
anthropology and political economy research and has clear applications in studies of human parallel evolution and adaptation more broadly
as well.
Like the concept of culture itself,
27
“biocultural”research has not been defined in just a single way, and there has been a notable lack of
agreement on exactly what constitutes biocultural research.
26,28
However, biocultural approaches broadly seek an integrated understanding of
the interactions and interrelations between human biology and the social and biotic environments that humans occupy, and how these
interactions impact well‐being. This framework doesn't prescribe a rigid set of analyses to be undertaken but ratheremphasizestheimportance
of incorporating multiple viewpoints, integrating qualitative and quantitative data, and analyzing the interconnections between biological
mechanisms and the biotic and social environments that shape human variation.
25
Some biocultural studies adopt a more formal statistical
approach, using regression analyses to model the impact of cultural traits (e.g., culturally ideal family traits, which were inferred through
ethnographic surveys) on various biological metrics (e.g., blood pressure, depressive symptoms, or other indicators of stress).
28
In other cases, a
general concept of a group's culture is used to contextualize the results of a biological analysis. For instance, instead of focusing just on genetic
correlates of disease and biotic environments, biocultural studies show how unequal power structures, structuralized poverty and racism, and
other sociopolitical processes also shape human biological variation and disease.
24,29
By considering the impacts of socio‐politico‐cultural
processes on human bodies in tandem with scans for genetic adaptation, we can glean a more nuanced and holistic understanding of human
biological variation worldwide.
To do this, biocultural frameworks should ideally be operationalized in adaptation research from the onset to shape questions asked,
hypotheses tested, data analyzed, and conclusions drawn. Researchers should be open to different viewpoints, collect qualitative data along
with quantitative genomic/physiological data, and think critically about how sociopolitical factors may be impacting the groups being studied.
Early in the research design process, researchers should have discussions with community members or descendent communities (for studies
involving ancestors) about the proposed research and aspects of their lives, lived experiences, and cultural practices that may be relevant to the
research topic, such as socioeconomic status and how that impacts daily lived experiences, historical or present‐day religious practices, familial
and marital relationships, practices around diet and healthcare, family stories, and family and/or group histories. These discussions could occur
through formal ethnographic interviews, written surveys, and/or informal conversations, ideally all planned and conducted in collaboration with
community partners and sociocultural anthropologists. These discussions will strengthen the resulting studies and enable communities to
become co‐creators of the scientific knowledge produced.
30
Previously published ethnohistorical and archaeological studies may also help shed
light on relevant biotic and social environments (though it is important to recognize that early ethnographic accounts may have been biased).
While qualitative data would preferably be gathered at the outset, to help shape the research design and hypotheses, these data
can be collected later in the research process, before conclusions are drawn, if necessary. The example in Figure 1, as well as the
examples discussed throughout this review, show how biocultural frameworks help clarify the interconnections between biology and
culture and produce a more complete and nuanced understanding of human evolution and biological variation.
FIGURE 1 Hypothetical biocultural framework for cold tolerance among circumpolar populations. Each box identifies different biotic and
sociocultural factors that may impact biological variation within these populations, with the arrows suggesting possible interactions and
connections among otherwise seemingly disparate data. Modeled after Rankin‐Hill.
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The commonness of this evolutionary process in humans illustrates
how, through a combination of natural selection acting on genetic
variation, a high degree of plasticity, and sociocultural mechanisms,
people can survive and thrive in almost every environment. In this
review, we discuss both the biological and cultural mechanisms
influencing parallel evolution in human populations—and their
interconnections—through four examples: malaria resistance, lactase
persistence, cold tolerance, and high‐altitude adaptation.
2|MECHANISMS LEADING TO PARALLEL
EVOLUTION
Parallel evolution occurs when natural selection acts on different
organisms or populations to maintain similar ancestral (existing)
genotypes and/or phenotypes, or to produce new convergent
genotypes/phenotypes. Here, we use the term parallel evolution to
encompass all forms of repeated evolution. The effects of this
process can be seen in the form of homologies (such as the bone
structure of bird and bat wings), homoplasies (such as prehensile tails
in monkeys from the Americas belonging to the divergent families
Atelidae and Cebidae), and shared genotypes/phenotypes (such as
those in human populations described below).
4,6,12
In humans, both
biotic and sociocultural factors influence when and how parallel
evolution occurs.
Parallel evolution can be observed at various biological levels. At the
most localized scale, it may involve single nucleotide changes within one
gene, where an adaptive mutation arises that affects just one or a few
nucleotides. In these contexts, one of two patterns of parallel change may
be observed in the genome: (1) the same nucleotides at the same locus
are at high frequencies (and, therefore, assumed to be under selection) in
two or more populations with separate demographic histories (Figure 2a),
or (2) different nucleotides at the same locus are at high frequencies in
each population, where distinct nucleotide mutations impact the
phenotype encoded by that gene in the same way (Figure 2b). Parallel
evolution can also occur at different genes that are part of the same
polygenic pathway, or part of the same or similar developmental and/or
physiological pathways (Figure 2c). Parallel evolution can, therefore, be
thought of as a (non)parallel continuum from perfectly parallel (Figure 2a)
to less‐than‐parallel (Figure 2b,c).
1
In human populations, parallel evolution is most often identified
through comparative research. Studies of genetics, development, or
physiology are conducted in populations thought to be experiencing
similar selective pressures, and the results compared to identify
shared or similar adaptations. Genetic approaches to identifying
adaptation usually rely on statistical tests that scan the genome for
signals of selection.
10,18
Some of these tests infer selection by
comparing allele frequencies between a population experiencing the
selective pressure and a related population not experiencing it. If an
allele is at high frequency in the population experiencing selection
but at low frequency in the related population, the allele is thought to
be an adaptation.
31
Other tests measure the length of haplotypes
(i.e., stretches of chromosomes inherited as a unit) in the population.
They infer selection on an allele when the surrounding haplotype is
significantly longer than other haplotypes in the genome as
haplotypes that contain beneficial alleles tend to be conserved and
broken up less often by recombination.
18,32
Once putatively selective
alleles are identified, functional in vitro experiments or physiological
FIGURE 2 Parallel evolution from genotype to phenotype. (a) Selection for the same nucleotide at the same locus in two studied
populations. (b) Selection for different nucleotides at the same locus that impacts the phenotype similarly. (c) Selection on different loci in two
populations, but at loci that are expressed as part of the same or similar biological pathways and thus have similar phenotypic effects.
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analyses are needed to determine the phenotypic effect of the
allele(s) if not already known. Additionally, physiological adaptation
studies measure phenotypes thought to be beneficial—and, there-
fore, adaptive—in a population living in a certain environment
compared to a nearby sister population inhabiting a different
environment. Differences in the phenotypes of interest in these
populations are inferred to be adaptations. If similar putatively
selected alleles and/or physiological phenotypes are seen across
multiple populations experiencing similar selective pressures, we
surmise that parallel evolution occurred. Importantly, while this
comparative approach is valuable for identifying putative cases of
parallel evolution, it could still be confounded by shared population
histories, where populations might exhibit similar phenotypes due to
shared ancestry rather than selection.
2
Methods commonly used to quantify parallel evolution in
nonhuman organisms could provide additional insight if applied to
human populations. One such method, phenotypic change vector
analysis (PCVA), quantifies parallelism and degrees of convergence/
divergence in pairs of populations that span some environmental
range (Figure 3).
1,12,33
If this method were applied to cold‐adapted
populations (Figure 3a), for instance, two phenotypes or other
quantifiable characteristics (e.g., allele frequencies or physiological
measurements, such as basal metabolic rate and thyroid function)
would be measured in multiple populations who inhabit cold climates
and in nearby sister populations who inhabit moderate climates. The
centroids (i.e., multivariate means) of the measurements of basal
metabolic rate and thyroid function would be calculated for each
cold‐and moderate‐climate population and plotted on a graph, with
the X‐axis representing basal metabolic rate and the Y‐axis repre-
senting thyroid function. Vectors would then be drawn to connect
the centroids of each cold‐and moderate‐climate sister population
pair, with the length of each vector, L, indicating the magnitude of
evolutionary change between environments. The amount of parallel-
ism in these changes between multiple population pairs can be
quantified in two ways. First, we can measure the angle, θ, between
two vectors (i.e., examining two population pairs). Perfect parallel
evolution is indicated by θ≅0°, with nonzero values indicating
differing amounts of (non)parallel evolutionary change between
population pairs (Figure 3a,b, top row).
1
Second, we can measure
the difference in magnitude of the vectors between multiple
population pairs (i.e., again examining at least two population pairs),
ΔL. This is seen in Figure 2a, where ΔLis found by subtracting L
b
from
L
a
. Perfect parallelism is indicated by ΔL≅0 (which is not the case in
Figure 3a). Thus, while PCVA appears similar to regression analysis, it
does not actually utilize linear equations when measuring vectors.
Additionally, if the ancestral environment and/or ancestral state of
the traits of interest are known, we can infer evolutionary divergence
and/or convergence using the graph created by PCVA. Evolutionary
divergence is indicated by vectors angling away from each other,
while convergence is indicated by vectors angling towards one
another (Figure 3b, bottom row). This example uses data from sister
populations to look at parallel evolution across environmental
FIGURE 3 Phenotypic change vector analysis (PCVA) for quantifying parallel evolution. (a) In this hypothetical example, parallel evolution is
quantified in two pairs of populations (pair a and pair b), where one population from each pair inhabits cold climates (circles) and the other
population inhabits nearby moderate climates (squares). The centroids (i.e., multivariate means) for two phenotypes (e.g., basal metabolic rate
and thyroid function) are plotted along the X‐and Y‐axes for cold‐climate population a, moderate‐climate population a, cold‐climate population
b, and moderate‐climate population b. The centroids of the population pairs are connected by a vector. The amount of parallelism between the
two pairs of populations is calculated by measuring the angle, θ, between the two vectors. Perfect parallelism is indicated by θ≅0°, with
deviations indicating (non)parallelism (as shown here). The difference in magnitude between the two vectors, ΔL, can also indicate parallelism,
with ΔL≅0 suggesting perfect parallelism (not shown). (b) Schematic of possible outcomes of charting phenotypic change in hypothetical
population pairs. The (non)parallel evolutionary continuum ranges from perfectly parallel (top left) to differing degrees of (non)parallelism (top
right). Differences between the starting and end points of the vectors can also indicate divergence or convergence of traits if the ancestral state
of the trait or the ancestral environment is known (bottom row). Figure adapted from Bolnick et al.
1
and Stuart.
12
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gradients, but the same approach could use data from ancient
individuals (ancient DNA or skeletal analyses) to quantify parallel
evolution through time.
1
While PCVA can provide important insights into the history of
parallel evolution in humans, it does have limitations. The method
requires several sister or ancestor‐descendent population pairs to
ensure that observations of parallelism are real and not false positives
due to random evolutionary chance. The traits chosen for analysis
also need to be independent to avoid biasing the results of the
analysis.
1
Finally, it should be noted that the angles and magnitude of
evolutionary change observed with this method may stem from many
different evolutionary processes, not just selection, or may have
unclear biological significance.
1
Nevertheless, PCVA may still provide
interesting insights into parallel evolution in human populations.
Parallel evolution in human populations can be directly impacted
by culture as the environments we occupy are extensively shaped by
culture. It is, therefore, important to consider socio‐politico‐cultural
contexts that influence human biologies as well. For instance,
socioeconomic status and religious practices can impact diet, access
to health resources, and exposure to biotic risks.
25
Population
histories are shaped by cultural norms of interpersonal interactions
and inter‐and intragroup dynamics. And migrations into new and
possibly harsh environments may stem from lack of resources,
interpersonal conflicts, and innumerable other culturally determined
contexts.
In each of the following examples of parallel evolution, we
highlight the genetic and physiological adaptations that have been
identified and discuss the cultural contexts impacting human
biological variation. We do not explore the population genetic
models or mathematical formulae underlying the discussed examples
in depth here as they are beyond the scope of this review and have
been covered elsewhere.
1,4,10,18,34
Instead, we focus on the broad
patterns of parallel evolution in humans and how cultural contexts
shape human evolution. In doing so, we show that combining cultural
considerations with genomics and physiological research produces a
more complete and nuanced understanding of human parallel
evolution.
3|MALARIAL RESISTANCE: PARALLEL
ADAPTATIONS DRIVEN BY AGRICULTURE
AND GLOBALIZATION
Malaria is a strong driver of natural selection in tropical, subtropical,
and temperate regions across the globe (Figure 4), with over 200
million cases and nearly half of a million deaths annually (of which
almost 70% are children).
35
Five species of parasites from the genus
Plasmodium cause malaria in humans: Plasmodium falciparum, P. vivax,
P. malariae, P. ovale, and P. knowlesi, with the first two (P. falciparum
and P. vivax) causing the most cases worldwide.
35
Mosquitoes from
the genus Anopheles, which thrive in warm climates, carry the
Plasmodium parasite and transfer it into human hosts when the
mosquito feeds on their blood. Plasmodium invades the liver and then
moves into the blood to infect blood cells, reproduce, and burst out
to infect new cells. New mosquitos pick up the parasite when feeding
on infected humans, allowing Plasmodium's life cycle to begin anew.
FIGURE 4 Map showing locations of the examples of parallel evolution reviewed here. This map depicts the general geographic distribution
of each trait but may not show the entire distribution of every adaptation.
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Malaria symptoms, including fever and other flu‐like symptoms that
can lead to death (especially in children), occur when the parasite
infects blood cells.
36
Numerous blood cell‐related adaptations have,
therefore, evolved to help humans resist malarial infection, including
sickle cell trait, thalassemias, and G6PD deficiency.
36–39
Notably, malaria only became common around 10,000 years ago,
and population genetic analyses show that adaptations for malarial
resistance arose around the same time.
37
This coincides with a major
cultural development in human history: the advent and spread of
agriculture in Africa and Southwest Asia.
38,40
With agriculture came
major environmental changes that allowed humans, mosquitos, and
Plasmodium parasites to all flourish. Farmers cleared forests to create
agricultural fields, which provided sunlight for breeding mosquitos,
and farming removed the absorbent topsoil, leading to more pools of
standing water where mosquitos breed. Farming also led to more
dense concentrations of people, providing easy meals for hungry
mosquitos and human refuse for additional breeding places. These
culturally driven changes produced larger mosquito populations that
transferred Plasmodium easily and frequently from infected to
noninfected humans.
39,41,42
Malaria spread into the Mediterranean and farther into Asia with
subsequent human migrations,
43
and colonialism and more‐recent
globalization spread the disease throughout the world's warmest
regions. While the presence of endemic malaria in the Americas
before European colonization is debated, three of the human
Plasmodium parasites (P. falciparum, P. vivax, and P. malariae) are
now endemic in the neotropics.
44
P. falciparum and P. malariae may
have been brought to the Americas by slavers during European
colonization, while immigrant tea farmers likely brought P. vivax from
East Asia (where it is most common today) during the 19th century.
44
The recent introduction of malaria to the Americas is supported by
genetic adaptation studies, which have identified only admixture‐
related adaptation after European colonization in Indigenous popula-
tions in the Americas.
44,45
More recent migrations have also
introduced malaria into places where it was likely never present
before, including India and parts of Latin America.
42
As malaria spread worldwide, alleles to resist malarial infection
evolved in parallel. Human malaria resistance adaptations are
numerous and varied, with even geographically proximate popula-
tions sometimes exhibiting different adaptations, highlighting the
complexity of malaria's impact on human evolution.
38
The vast
majority of adaptations impact how well Plasmodium survives in the
bloodstream, where malarial infection has the most detrimental
effects on human health.
36–38
Many of the mutations thought to be
protective against malaria are found within the α‐and β‐globin genes
HBA and HBB, and affect the shape and structure of the hemoglobin
protein in red blood cells. Some mutations involve changes to only a
single codon, such as with the HbS, HbC, and HbE alleles of the HBB
gene, whereas others entail the deletion or inactivation of the entire
gene. For example, the HbS (“sickle‐cell”) allele of the HBB gene
exhibits a single nucleotide change (A > T) in codon 6, causing the
sixth amino acid in the β‐globin chain to be a valine instead of
glutamic acid. This single difference causes hemoglobin molecules to
change shape after releasing the oxygen they carry from the lungs to
different parts of the body, deforming the red blood cell and making
it look crescent or “sickle”shaped. If an individual is homozygous for
this recessive variant, they may develop sickle‐cell anemia during
times of high oxygen consumption, when much hemoglobin become
deoxygenated in a short period of time. This can have devastating
consequences and often leads to premature death. Similarly,
α‐thalassemia, which is caused by deletion or inactivation of the
HBA gene, and β‐thalassemia, caused by deletion(s) or inactivation(s)
in the HBB gene, affect the production of the α‐and β‐globin
molecules, respectively, that make up hemoglobin. When the α‐or β‐
globin proteins are absent or present in reduced quantities, red blood
cells become unstable and die prematurely, negatively affecting that
person's health. However, these alleles remain at high frequencies in
regions with endemic malaria because they are protective against the
disease.
36–38
The precise mechanisms by which these hemoglobin‐related
adaptations confer protection are still debated, but likely involve
more effective removal of diseased red blood cells from the body.
36
Because heterozygotes gain this benefit without suffering from the
anemias that afflict homozygotes, the variants persist (a phenomenon
known as balanced polymorphism). These adaptive hemoglobinopa-
thies provide a clear example of parallel evolution in human
populations, with different mutations in the same gene families
being selected for because they produce similar phenotypic
outcomes in response to the same selective pressure. In fact, there
is even evidence for the repeated independent evolution of some
specific alleles. The HbS allele is found on different haplotypes in
African, Southwest Asian, and Indian populations, indicating that the
HbS mutation has arisen independently at least three times.
37
Adaptations that do not involve hemoglobin have also evolved
that may help humans resist malarial infection. G6PD deficiency, an
enzymopathy that leads to elevated levels of oxidative stress in red
blood cells, is another balanced polymorphism in regions with
endemic malaria.
40
More than 140 different mutations in the G6PD
gene on the X‐chromosome lead to G6PD deficiency, with different
alleles found at high frequencies in Africa, Asia, and the Mediterra-
nean.
40,46
Individuals with G6PD deficiency show decreased severity
of malarial infection,
46
likely due to the buildup of oxidative stress
within G6PD‐deficient red blood cells. Oxidative stress has been
shown to slow the parasite's reproduction and cause infected cells to
burst before the parasite can fully mature.
36,46
Notably, cultural practices such as the cultivation and consump-
tion of fava beans may affect the prevalence and impact of G6PD
deficiency in malaria‐endemic regions. When individuals with G6PD
deficiency consume fava beans, they often experience favism, a
condition in which blood cells rupture due to the accumulation of
oxidative damage.
46
Despite the risks of favism, consumption of fava
beans in regions with high frequencies of G6PD deficiency is
common.
46,47
Katz and Schall
47
developed a biocultural evolutionary
explanation for this surprising pattern, suggesting that fava bean
consumption may be beneficial for non‐G6PD‐deficient individuals
because it produces moderate oxidative stress in red blood cells,
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making them inhospitable for the Plasmodium parasite, similar to what
occurs in G6PD‐deficient red blood cells, and decreasing the severity
of malarial infections. Thus, balancing selection may act to maintain
both fava bean consumption and G6PD deficiency despite the
negative effects of favism because both protect against malarial
infection.
As these genetic adaptations evolved and spread across the
globe, other cultural strategies for mitigating the detrimental effects
of malaria also developed. For example, through antiquity and into
more recent times, Mediterranean elites would leave their marshy
farmlands during summer, when malaria was most prevalent, to
escape the “bad air”that caused the disease.
42,43
Additional practices,
such as the use of traditional insect repellants, alkaline soaps that
destroy mosquito breeding areas, and mechanical barriers such as
clothing, heavy blankets, and netting, have been used to further
decrease the prevalence of malaria.
48
Quinine, derived from South
American cinchona bark, has also been widely adopted as an anti‐
malarial drug since 17th century Jesuit priests brought the bark back
to Europe (although we note that there is ongoing debate about
whether Indigenous groups in South America were the first to use the
bark as a treatment for malaria, reflecting disagreement about
whether malaria was endemic in the Americas before European
colonization).
44
Today, malaria elimination efforts focus on providing
insecticide‐treated mosquito nets to affected regions as well as
making preventative medications, rapid diagnostic tests, and treat-
ments widely available.
35
Biocultural frameworks are critical for
understanding the diverse biological, sociocultural, and technological
forms of malarial resistance that have emerged repeatedly in human
populations.
4|LACTASE PERSISTENCE: MORE
PARALLEL EVOLUTION DRIVEN BY
AGRICULTURE
Although most mammals are unable to digest milk after weaning,
some humans are an exception. The ability to drink milk without
discomfort in adulthood is known as lactase persistence (LP), where
the enzyme lactase—which helps digest the main carbohydrate in
milk, lactose—is continually produced during an individual's lifetime
rather than decreasing after infancy. The human body cannot
efficiently digest lactose without lactase, and people who are lactase
nonpersistent (LNP) suffer from lactose intolerance that causes
severe bloating, cramping, and bowel issues when milk is con-
sumed.
49
Early European doctors and researchers assumed LP was
the norm among humans as LP is exceedingly common in European
populations,
50
with frequencies reaching 80% in Northern European
groups.
49
However, except in certain parts of Europe, Africa, and
Asia, most humans are LNP and cannot digest milk after early
childhood (Figure 4).
51
LP was originally thought to be a simple Mendelian trait with a
single causative single‐nucleotide polymorphism (SNP), but the
genetic basis actually varies from region to region. Functional
analyses have shown that LP is caused by one or more of five
SNPs in the MCM6 gene, which regulates the expression of the
LCT gene that controls lactase production.
52,53
Other alleles in the
MCM6 gene have also been identified that may cause LP, but they
have not yet had their function validated experimentally.
54
Nevertheless, it seems clear that the genetics of LP is more
complicated than once thought. One allele, known as −13910*T, is
mostcommoninEuropeandSouthwest/CentralAsiaandin
populations with recent ancestors from these regions (e.g., in the
Americas).
49,52,55
Population genetic studies suggest that this
allele arose in Eastern Europe/Western Asia and spread with
farmers and pastoralists migrating into Europe and farther east
into Asia.
56
Three other MCM6 alleles, involving mutations at
−14010*C, −13915*G, and −13907*G, have evolved indepen-
dently in Southwest Asia and South, East, and West Africa,
providing clear evidence of parallel evolution for LP.
49,53
Biocultural models are essential for understanding why LP has
evolved repeatedly and is at such high frequencies in some
populations while being rare or nonexistent elsewhere. Notably, the
global distribution of LP matches where animal domestication arose
~10,000 years ago during the agricultural revolution and pastoralist
societies came to rely on meat, milk, and other products from
domesticated animals.
49,57
Local variation in LP is also generally
correlated with subsistence strategy: some neighboring groups in
Africa and in Southwest Asia exhibit distinct patterns of LP, with
pastoralist/herder groups exhibiting high frequencies of LP and
nearby farmers showing no LP.
49,50,58
Two biocultural hypotheses have been proposed to account for
the overlapping distributions of LP and pastoralism. First, the culture‐
historical hypothesis suggests that as populations began to rely on
domesticated animals, it became increasingly beneficial to be able to
digest their nutritious milk. Thus, as dairying spread throughout
Southwest and Central Asia, Eastern Africa, and Europe, selection
acted to increase the frequency of LP.
49,59
Alternatively, the reverse‐
cause hypothesis suggests that dairying only began in locations where
LP was already common.
57,60
Data from archaeological and genetic
research primarily support the culture‐historical hypothesis.
56
Popu-
lation genetic analyses suggest that selection favoring LP alleles
began between ~1,625 and 11,200 years ago depending on the
population being studied.
49
Archaeological evidence indicates that
animals were being used for their milk mostly before these increases
in the frequency of LP. In the Early Neolithic (~10,000 years ago),
animal rearing practices had begun to change in ways suggesting that
animals were being used for their milk, such as by weaning calves
early (so any additional milk produced by mothers may have been
consumed by humans) and keeping females alive longer before killing
them for their meat.
61,62
Further, milk fat residues on pottery
fragments from this time indicate storage of milk.
57,58,62–64
Finally,
ancient DNA studies in Europe also show negligible frequencies of LP
alleles in Mesolithic through Bronze Age populations (~15,000–3,000
years ago), and indicate that these alleles only became exceedingly
widespread by the Middle Ages.
49,64,65
Altogether, this evidence
suggests that LP evolved in parallel after dairying became common.
8
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Notably, in some cases, cultural practices do not align well with
the presence or absence of LP. For example, some populations in East
Africa continue to produce lactase through adulthood, but do not
consume milk or exhibit any of the known adaptive alleles in MCM6
or LCT.
49
There are also examples of groups, particularly in Central
Asia, that consume milk but have very low frequencies of LP.
49
Both
cultural practices and biological processes likely influence these
disparate patterns. For instance, people who are LNP can mitigate the
impacts of lactose intolerance by either drinking milk in small
quantities or by processing it to reduce the amount of lactose
present (i.e., cheese‐and yogurt‐making).
49,50,66
Additionally, the
physiology of milk digestion may be more complex than just the
persistence of lactase activity into adulthood. The majority of issues
for LNP individuals occur when lactose reaches the colon, where
colonic microbiota digest the carbohydrate. In groups where milk
drinking is common but LP frequencies are low, it is possible that
these colonic bacteria are more abundant or more efficient at
breaking down lactose.
66
Thus, genetic adaptations, cultural innova-
tions, and microbiome plasticity likely all contribute to LP variation
among human populations.
5|COLD TOLERANCE: SURVIVAL AND
THRIVING ACROSS CIRCUMPOLAR
ENVIRONMENTS
As humans migrated out of Africa and around the world, they
encountered diverse environments, including the freezing cold
climate of the Arctic and sub‐Antarctic (Figure 4). This harsh
environment exerts strong selective pressures on human populations
because extreme cold can harm the human body and prevent
vegetable foodstuffs from growing, limiting people to an almost
exclusively animal‐and fish‐based diet. Nonetheless, people have
inhabited northern and southern circumpolar regions for the past
~46,000 and ~12,000 years, respectively,
67,68
reflecting both
biological and sociocultural responses that have allowed diverse
populations to survive and thrive in circumpolar regions (Figure 5).
Biologically, humans exhibit short‐term, reversible physiological
responses to cold (acclimations) as well as genetic adaptations that
affect the physiology of populations who have lived in frigid
environments for generations (Figure 5). When exposed to cold
temperatures, a number of physiological processes are initiated to
maintain the body's homeostasis and protect vital internal organs.
Involuntary shivering (also called shivering thermogenesis) produces
heat to raise internal body temperature.
69,70
Vasoconstriction
narrows blood vessels in the hands and feet, redirecting blood to
vital organs in the core where less heat will be lost.
67,69,71
Over time,
if cold exposure persists, vasoconstriction will be temporarily
reversed: cold‐induced vasodilation brings blood and heat back to
the extremities to reduce the chance of cold injury such as frostbite,
and increasingly efficient cycles of vasoconstriction and vasodilation
ensue.
67
After prolonged or repeated exposures to cold, the body
becomes habituated and requires less shivering and less extreme
cycles of vasoconstriction/vasodilation to maintain homeostasis.
Metabolic acclimations to cold also include nonshivering thermo-
genesis, where the body breaks down stored brown fat tissue to
generate heat.
69,70
Insulative acclimation occurs as well, leading to
increased subcutaneous fat layers.
69,70
In sustained cold temperatures, people also behave differently to
produce and maintain body heat. While some of these behaviors are
universal (e.g., voluntary movement/exercise to generate heat),
67
many are shaped by the sociocultural contexts in which people live.
FIGURE 5 Progression of the human body's adjustments to extreme cold conditions
BALENTINE AND BOLNICK
|
9

Techniques for making fire for warmth have been passed down from
generation to generation for millennia. People have long constructed
heated and/or well‐insulated shelters to provide protection from the
cold, with shelter types varying by group.
71
For example, before
European colonization, groups in southern South America con-
structed houses of wood covered by insulative guanaco (Lama
guanicoe, a relative of the llama) skins, with fires constantly burning
within.
72
In contrast, some groups in Alaska lived in skin tents during
the summer months and moved into insulated semisubterranean
homes in the winter.
73
Well‐insulated and waterproof clothing is also
important, and also varies from group to group. Groups in southern
Patagonia and Tierra del Fuego, for example, wore guanaco‐fur capes
and pants and rubbed seal fat onto their skin to further protect
against cold and wind, while people in the Arctic often wore caribou‐
fur clothing and water‐resistant raingear made from animal guts.
72,73
Populations that have inhabited cold environments for genera-
tions have also evolved genetic adaptations in response to cold
stressors that, while showing some similarities among groups, offer
examples of the less‐than‐parallel end of the parallel evolutionary
continuum. Specifically, selection has been inferred in different genes
that influence similar physiological pathways across multiple Arctic
groups (Figure 2c). For instance, selection at genes related to fat
tissue differentiation, believed to play a role in nonshivering
thermogenesis, has been inferred at the KCNH1 gene in the Alaskan
Iñupiat
74
and at the WARS2 and TBX15 genes in the Greenlandic
Inuit, the latter two of which were derived from Denisovan
introgression.
75,76
Parallel adaptations have also been inferred at
genes that modulate fatty acid metabolic pathways in Indigenous
Siberians, Greenlandic Inuit, and the Nunavik Inuit from Canada,
which may be tied to the need to break down fat more efficiently due
to their high‐fat fish‐and animal meat‐based diets. The putatively
selected genes vary widely across these groups and include the
CPT1A, LRP5, THADA, CPNE7, ICAM5, HADHA, HADHB, PLA2G2A,
PLIN1, ANGPTL8, and FADS1/2 genes.
75,77–81
Adaptation studies
have not yet been reported in southern South American groups,
which could elucidate even more cold‐adapted genes. The identifica-
tion of adaptation in so many different genes within similar
physiological pathways could be a result of studies using different
statistical tests to identify selection and some studies having
incomplete genomic data, but it might also reflect complex polygenic
adaptation, especially because different forms of sociocultural
buffering in cold environments could have influenced the direction
that natural selection has taken in different populations.
While few genetic studies to date have explored this possibility,
instead using a more strictly biological lens to investigate the
mechanisms of survival and thriving at high latitudes, some
physiological studies suggest that the interplay between biology
and sociocultural factors may be very important. For example,
multiple studies have found that Indigenous peoples across circum-
polar North America, Asia, and Europe show an elevated basal
metabolic rate (BMR) compared to noncircumpolar popula-
tions.
13,67,82,83
(BMR studies have not yet been reported in sub‐
Antarctic South America, so it is not known if populations there
exhibit this same adaptation). BMR is the amount of energy used by
the body at rest, and this physiological adaptation helps people
maintain homeostasis in extremely cold environments by generating
more metabolic heat.
82‐84
BMR is modulated by thyroid function, and
biocultural studies of the Indigenous Yakut (Sakha) people of Eastern
Siberia show that both seasonal changes (i.e., colder temperatures
during winter months) and socioeconomic factors affect thyroid
activity, with individuals who practice more traditional lifestyles (e.g.,
being more active and consuming foraged foods) showing the largest
increases in thyroid function, likely resulting in higher BMR and
metabolic heat production.
84
A similar pattern has been observed in
Greenlandic Inuit populations practicing traditional lifestyles.
85
These
studies show that both biotic and sociocultural environments matter
if we want to understand human biological variation and adapta-
tion.
67,84,85
If we do not consider the sociocultural and political
contexts in which people live, we may miss important factors
influencing human genetic variation and not fully understand the
pattern of genetic adaptation in diverse populations living in
circumpolar environments.
6|HIGH‐ALTITUDE ADAPTATION:
PARALLEL EVOLUTION ACROSS THREE
MOUNTAIN RANGES
While humans have lived in high‐altitude environments (i.e., at
elevations of 2500 m [8200 ft] or higher) for millennia, they face a
variety of challenges in these locations, including low oxygen levels,
cold temperatures, and increased UV radiation.
86
In particular,
because barometric pressure is lower at high altitude than at sea
level, fewer air molecules are present, and significantly less oxygen is
inhaled with each breath. This creates a risk of hypoxia (insufficient
oxygen to sustain bodily functions) and acute mountain sickness (a
condition characterized by fatigue, nausea/vomiting, headache, and
breathlessness, which may subside over time).
71,86,87
In the short term, to compensate for low oxygen levels when
visiting high altitudes, people breathe more rapidly (hyperventilation)
to bring in more oxygen and increase their heart rate to help
distribute it throughout the body.
69
After days or weeks in a hypoxic
environment, activation of the hypoxia inducible factor (HIF) genetic
pathway causes a cascade of proteins to be produced that promotes
red blood cell formation and increases hemoglobin concentration to
help transport oxygen to tissues throughout the body.
9,86
Notably,
these physiological changes are not permanent and can be reversed if
a person subsequently returns to lower elevations.
In contrast, genetic, physiological, and developmental adapta-
tions to hypoxia have evolved over time in populations that have
resided at high altitudes for many generations. Research into these
adaptations has focused primarily on three regions of the world: (1)
the Qinghai‐Tibetan Plateau in the Himalayas, which has been
occupied for at least ~35,000 years; (2) the Andean Altiplano,
occupied for ~11,000 years; and (3) the Simien Plateau in Ethiopia,
which has been occupied by two ethnic groups, the Amhara and the
10
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BALENTINE AND BOLNICK

Oromo, for ~5,000 and ~500 years, respectively (Figure 4).
86,87
Archaeological evidence suggests that people have lived in other
high‐altitude regions, such as the North American Rocky Mountains,
for several thousands of years,
88
but these populations have not
been studied to the same extent, or in some cases, at all.
In Tibet, Ethiopia, and the Andes, highland populations exhibit
parallel adaptations to high altitude, although the complexities of
parallel evolution are evident. Five genes (EDN1, EDNRA, EGLN1,
NOS1, and VEGFA) involved in the HIF pathway exhibit putatively
adaptive variants in all three regions, though the dozens of specific
SNPs in these genes differ among populations (Figure 2b).
87
Another
27 genes also exhibit putatively adaptive alleles that are shared by
highland populations in two of the three areas
87
and studies
searching for shared signals of selection among these populations
have inferred selection at even more loci as well.
2,89
Populations in each region exhibit some shared and some unique
adaptive physiologies. Researchers have identified high‐altitude
adaptive physiologies by comparing multigenerational high‐altitude
populations with nearby related lowland populations (who either
reside at low altitudes or have only recently migrated to high
altitudes). The most commonly identified adaptations in multigenera-
tional high‐altitude populations have been (1) increased birthweight
compared to lowlanders living at high altitudes (because the hypoxic
environment can be detrimental to fetuses and newborns, but
developmental adaptations have mitigated this in high‐altitude
groups), (2) increased resting ventilation rates (leading to more
oxygen inhaled), (3) increased hemoglobin production (to help
transport oxygen throughout the body), and (4) higher arterial oxygen
concentrations (which aid in delivering oxygen to vital tissues).
86,87
However, there is variation in these adaptive physiologies among
high‐altitude populations with only some shared phenotypes and no
high‐altitude population exhibits all four adaptations.
For instance, increased birthweights have been observed in high‐
altitude populations in both the Andes and the Himalayas (no studies
of birthweight in relation to altitude have been reported in Ethiopian
populations). Andean and Ethiopian populations also both show
increased hemoglobin production and arterial oxygen concentrations,
while Himalayan populations do not. Lastly, Himalayan populations
exhibit increased resting ventilation rates compared to lowland
groups, while Andean populations show no such differences (again,
no studies of this phenotype have been reported in Ethiopian
groups).
9,87
This variation in high‐altitude adaptations among
populations living in these three regions offers a clear illustration of
the complexities of parallel evolution in human populations. Further
research is needed to better clarify the whole suite of genetic,
physiological, and developmental changes associated with high‐
altitude adaptation.
Importantly, while most high‐altitude adaptation research has
focused on biotic environmental pressures, it should be noted that
sociocultural pressures play a significant role in shaping phenotypes
at high altitude as well. High‐altitude researchers in the 1960s–70s
observed slower growth and shorter statures in Indigenous peoples
from the high‐altitude Andes compared to nearby lowlanders, and
attributed these phenotypes to developmental plasticity and/or
genetic adaptation in response to the stresses of a hypoxic
environment.
29,90,91
However, biocultural research beginning in the
1980s showed that these Indigenous populations were (and often
still are) severally socially marginalized and had limited access to
nutritional foods, which was likely responsible for the stunted growth
and shorter stature.
91–93
This conclusion was further supported by a
recent longitudinal study of growth and development in children from
the high‐altitude town of Nuñoa, Peru, from 1964 to 2015.
92
In this
study, researchers found that sociopolitical factors had directly
influenced stature in this population. Between the 1960s–1990s,
when numerous periods of sociopolitical upheaval maintained
marginalization and malnutrition, there was little change on average
height. However, from the 1990s–2015, they observed an increase in
stature that coincided with social development, including improved
economic conditions and healthcare access.
92
Thus, while hypoxia
has a significant impact on human biologies, factors like social
marginalization and malnutrition matter as well. Furthermore,
sociocultural practices have also been developed that help mitigate
the effects of hypoxia. The consumption of coca leaves (Erythroxylum
coca and E. novogranatense), for example, has been practiced in the
Andes for at least 3,000 years.
94
Chewing the leaves or drinking tea
made from them is traditionally used to alleviate mouth or tooth pain
and gastrointestinal issues, among other ailments, and to help
mitigate the effects of hypoxia.
95
While studies on the efficacy of
coca leaves as treatment for hypoxia have yielded mixed results,
96,97
their cultural significance and use as a traditional medicine in the
Andes remains important.
71,95
Together, these examples illustrate the
importance of sociocultural and political contexts and the need to
incorporate biocultural perspectives into adaptation research as not
all phenotypes are due to adaptive evolution.
7|FINAL THOUGHTS AND FUTURE
DIRECTIONS
Through the above examples, we can see how biocultural approaches
provide a more thorough picture of human parallel evolution and
human adaptability more broadly. Myriad biological and sociocultural
processes work together to permit humans to survive and thrive in
similar environments around the globe. Humans are highly plastic and
able to acclimate to new environments relatively quickly, and while in
these new environments, develop cultural technologies that make it
easier for them to survive and stay for generations. Over time, natural
selection also acts on shared ancestral genetic variation or new
mutations, leading to parallel genetic adaptations. Furthermore, the
development and sharing of new cultural technologies can create
new environments with new selective pressures, which may also
push different human groups to evolve in parallel directions.
Biocultural approaches are, therefore, critical: if we examine human
parallel evolution from only a biological perspective, we miss how
human culture acts to both mitigate and create selective pressures,
which has significant impacts on human biological variation.
BALENTINE AND BOLNICK
|
11

Many other instances of human adaptation and parallel evolution
could benefit from biocultural analyses as well, including heat
tolerance,
71
variations in height (such as the shorter stature observed
in rainforest populations),
9
responses to infectious diseases other than
malaria,
48
and variation in skin pigmentation (e.g., darker skin tones tend
to be found in people living at low latitudes near the equator, while
lighter skin is prevalent at higher latitudes).
8,65,71
Variationinskin
pigmentation is an especially important topic for future biocultural
research.Skincolordifferenceshavetakenongreatsocialsignificancein
a world shaped by European colonialism and this instance of parallel
evolution affects populations across the globe. The latitudinal gradients
we see are an adaptive response to UV radiation, with dark skin evolving
at low latitudes to prevent UV‐induced damage to DNA and folate
(needed for normal fetal development) and light skin evolving repeatedly
at higher latitudes to permit sufficient UV exposure to allow vitamin D
synthesis.
71,98
However,moreattentiontoskincolorvariationasan
inherently biocultural phenomenon is needed.
99,100
Cultural practices,
subsistence strategies, and biocultural processes such as migration have
all likely influenced skin pigmentation over the course of human history.
Cultural preferences for lighter or darker pigmentation and social norms
regarding clothing, occupation, and behavior all shape how much UV
exposure people get. Cultural developments like the spread of farming
mayhavealsoinfluencedthestrengthofselectiononskincolor.Some
hunter‐gatherers in Europe continued to exhibit dark skin even after
millennia of living at higher latitudes,
101,102
suggesting that they did not
face strong selective pressures for lighter skin pigmentation. Further-
more, genetic variants for light skin do not seem to have become
widespread until after farming became common.
65,101
It could, therefore,
be hypothesized that selection for lighter skin was tied to this cultural
shift, with selective pressures increasing only after farming became the
main subsistence strategy and diets shifted away from fish that were
high in vitamin D, similar to what has been hypothesized for some
Indigenous groups in the Americas.
102,103
Future biocultural studies
should investigate this possibility further.
While we have focused here on the application of biocultural
theory, other analytical frameworks can provide further insight into
human biological variation and adaptation as well. Developmental
and phenotypic plasticity, as well as epigenetic processes, should be
further investigated to clarify how they help humans acclimate to
new environments before genetic adaptations arise. Structural
variants, including large chromosomal deletions and rearrangements,
are an understudied part of our genome that likely also shape human
adaptation.
104
Finally, theoretical approaches such as niche construc-
tion theory and the extended evolutionary synthesis lend additional
lenses through which the complexities of human biology can be
studied.
21,27,57,105,106
These approaches provide frameworks for
considering how humans shape their environments to be more
habitable, allowing researchers to simultaneously consider how
human biologies are impacted by the environments that we are in
turn constantly manipulating.
Incorporating biocultural and other theoretical approaches into
rigorous studies of human adaptation is a big endeavor. For early
career researchers, it can be extremely daunting to learn new
analytical methods for studying biological variation while simulta-
neously seeking a broad understanding of the sociocultural mecha-
nisms at work in the populations being studied. Even at later career
stages, it can be difficult and time‐consuming to integrate diverse
theoretical frameworks and rigorous new analytical methods.
107
Transdisciplinary collaborations across the social and biological
sciences, as well as direct collaborations with the populations being
studied, are thus necessary for this kind of research to happen.
Despite the difficulties, biocultural approaches are crucial if we are to
glean a more accurate and nuanced understanding of human
adaptation.
ACKNOWLEDGMENTS
We thank Daniel Bolnick for discussions that influenced our thinking
about parallel evolution. We would also like to thank the participants
in the original research studies cited here that support this study.
Christina M. Balentine was supported by a National Science
Foundation Graduate Research Fellowship Program fellowship during
the preparation of this manuscript.
CONFLICT OF INTEREST
The authors declare no conflict of interest.
DATA AVAILABILITY STATEMENT
Data sharing is not applicable to this article as no data sets were
generated or analyzed during the current study.
ORCID
Christina M. Balentine http://orcid.org/0000-0001-7548-8790
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AUTHOR BIOGRAPHIES
Christina M. Balentine is a National Science Foundation
Graduate Research Fellow and PhD candidate in the Department
of Integrative Biology at the University of Texas at Austin. Her
research takes a critical biocultural approach to the study of
genetic adaptation to biotic and social environmental pressures in
human populations.
Deborah A. Bolnick is a Professor in the Department of
Anthropology and Institute for Systems Genomics at the University
of Connecticut. Her research in anthropological genetics and
biological anthropology explores how sociopolitical forces, historical
events, and social inequalities shape human genomic and epige-
nomic diversity, and human biology more broadly.
How to cite this article: Balentine CM, Bolnick DA. Parallel
evolution in human populations: A biocultural perspective.
Evolutionary Anthropology. 2022;1‐15.
https://doi.org/10.1002/evan.21956
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