The steps of chemical evolution that proceed the steps of biological evolution. (a) A simplified view of the series of quasi-biotic layers leading to the production of the first rudimentary functional cell (phospholipids, RNA strands, liposome cell membrane and DNA molecule shown are not on a mutually correct scale). (b) A simple rudimentary organism cannot arise in an extreme environment, and consequently, no native extremophile can evolve there. (c) An imported extremophile living in an extraterrestrial extreme environment even though its ancestors would not be able to live there. (d) The emergence of a 'hopeful monster,' an extremotolerant organism that can withstand the environment in a single improbable leap. Credits: bacterial images adapted from Mirumur, 2011, 2014.

Contexts in source publication

Context 1

... this realistic constructed scenario, it was discussed that an extremophile, as seen in Figure 1c, but not a mesopohile, as seen in Figure 1b, may exist in an extreme environment on a solar system body. However, interestingly, if an extremophile in principle can exist in that environment, then chemical evolution may also, even if the step between an extremophile and chemical evolution, a mesophile, cannot ( Figure 2b). For our purpose, it is clarified here that by the term extreme environment is meant an environment in which at least one known terrestrial extremophile is capable of living here-and-now. ...

Context 2

... step between a mesophile and an extremophile requires a series of adaptations, as shown in Figure 1a. However, the step between a mesophile and chemical evolution also requires a series in which the assembly of cellular components occurs, as shown in Figure 2. If the environment has always been extreme, then chemical evolution will not be able to complete the assembly of a native cell and consequently no extremophiles. ...

Context 3

... not surprisingly lead to a biosignature. However, the situation illustrated in Figure 2a,c. can more subtly lead to a chemosignature and eventually to a biosignature. ...

Context 4

... occurred in a series of layers with increasing extremity or environmental stressors. However, it is also the case that before the emergence of the first functional cell, before biological evolution begins in this first layer in Figure 1a, there are a number of layers with quasi-biotic steps in which the assembly of a cell through several layers occurs; namely, chemical evolution, as seen in Figure 2. ...

Context 5

... it can be hypothesised that a significant but possible deviation is possible where a cell on the boundary between progenote and autonomous cells achieves extremotolerant capacities that barely allow it to survive the extreme environment (Figure 2d). While mesophiles and extremophiles have their optimal growth conditions in their local environments, an extremotolerant organism differs from them in being able to tolerate an environment but not thrive in it. ...

Context 6

... not surprisingly lead to the existence of a biosignature in a world. However, the situation illustrated in Figure 2a,c. leads more subtly to a chemosignature and eventually to a biosignature, which provides conceptual tools that allow us to clarify on a deeper level what is meant by habitable worlds. ...