Hominin increase in cranial capacity, courtesy of Luke Jostins
A few years ago a statistical geneticist at Cambridge’s Sanger Institute, Luke Jostins, posted the chart above using data from fossils on cranial capacity of hominins (the human lineage). As you can see there was a gradual increase in cranial capacity until ~250,000 years before the present, and then a more rapid increase. I should also note that from what I know about the empirical data, mean human cranial capacity peaked around the Last Glacial Maximum. Our brains have been shrinking, even relative to our body sizes (we’re not as large as we were during the Ice Age). But that’s neither here nor there. In the comments Jostins observes:
The data above includes all known Homo skulls, but none of the results change if you exclude the 24 Neandertals. In fact, you see the same results if you exclude Sapiens but keep Neandertals; the trends are pan-Homo, and aren’t confined to a specific lineage….
The Pith: This post explores evolution at two different scales: the broad philosophical and the close in genetic. Philosophically, is evolution a highly contingent process which is not characterized by much replication of form and function? Or, is evolution at the end of the day aiming for a few set points which define the most optimal fitness positions possible? And how do both of these models relate to the interaction across genes, epistasis? In this post I review a paper which shows exactly how historical contingency could work through gene-gene interactions on the molecular genetic scale.
Imagine if you will a portal to another universe which you have access to. By fiat let’s give you a “pod” which allows you to move freely throughout this universe, and also let’s assume that you can travel fast enough to go from planet to planet. What if you see that on all the planets there’s a sludgy living “goo” of some sort? To complexify the issue imagine that upon further inspection the goo is divided between a predominant photosynthetic element, and “parasitic” heterotrophs. But aside from these two niches there’s little diversity to be seen in this cosmos. The “climax ecology” of all the planets resemble each other, in case after case convergent evolution toward the one-morphology-to-out-fit-them-all. We could from these observations construct a general theory of evolution which deemphasizes the role of contingency. In other words, there are broad general dynamics which shape and prune the tree of life in this hypothetical universe so that there is always a final terminal steady-state of the most fit morphology.
A model of evolution as a process of very general principles which converges upon a small finite range of optimal solutions has been promoted by paleontologists such as Simon Conway Morris. Stephen Jay Gould was a famous expositor of the inverse position, which emphasized chance and contingency. Gould’s suggestion was that if you ran the evolutionary experiment anew the outcomes each time would likely differ. In The Ancestor’s Tale Richard Dawkins leans toward the former position, insofar as he does assent to the proposition that evolutionary dynamics do inevitably forward certain broad trends, irrespective of the specific historical sequence of states antecedent to the terminus. More fanciful and speculative extrapolations of this logic are used to justify the ubiquity of a humanoid morphology in science fiction. The theory goes that a bipedal organism whose upper limbs are free to manipulate tools is going to be the likely body plan of intelligent aliens (though they will also have easy to add nose frills and such).