From genetics, I think the LCA was a chimpanzee that became isolated in an aquatic habitat


Allan Krill
 

Excerpt from the intro chapter to the book Chimpanzees and Human Evolution, by Muller et al. 2017. (My bold-italics for the last 3 sentences.)

The idea that modern chimpanzees might be similar to the LCA is frequently dismissed out of hand, because of the long time span involved. In a typical comment, Henry Gee insisted that we should not cast chimpanzees in the “role of Our Ancestors,” because they “have been evolving away from our common ancestor for precisely as long as we have” (2013: 714). This sentiment, with its underlying assumption that millions of years of evolution must have altered the chimpanzee lineage as much as they have the human one, is now widely shared.2  Robert Sussman wrote that “chimpanzees have been evolving for as long as humans and gorillas, and there is no reason to believe that ancestral chimps were highly similar to present-day chimps” (2013: 103). Meredith Small’s variant (1993: 128) was more measured: “Although we’d like to use chimps as models for our distant forebears, we often forget that they evolved down their own path, shaped physically and behaviorally by pressures slightly different from those our ancient ancestors experienced” (for more or less identical statements, see Ehrlich 2000: 166; Zuk 2013: 41). 

The problem with these arguments is that molecular evolution is not the same as phenotypic change. And even in closely related species, rates of morphological or behavioral change can differ substantially over time. Darwin was aware of this latter point, as illustrated by the quotation that opens this chapter. Pilbeam and Lieberman (this volume) observe that multiple primate lineages are morphologically conservative over extended periods of evolutionary time. The gibbon (Hylobatidae) radiation, for example, has been dated to around 7 Ma, putting it close in time to the Pan / Homo divergence (Israfil et al. 2011). Few would contend, however, that the common ancestor of the fourteen-plus gibbon species alive today looked very different from a modern gibbon, on the basis that gibbons have been “evolving for as long as humans and gorillas.”3 

The inference that chimpanzees are also a conservative species, little changed since their split from the hominin line, is based on careful analysis and straightforward logic (Pilbeam 1996; Wrangham and Pilbeam 2001; Pilbeam and Young 2004). Chimpanzees and gorillas are extremely similar morphologically, to such an extent that for decades they were considered to be monophyletic, fundamentally size variants of the same animal. Genetic data, however, are unambiguous in showing that chimpanzees and humans are more closely related to each other than either is to the gorilla. This points to one of two conclusions: the extensive similarities between chimpanzees and gorillas represent evolutionary convergence, or the last common ancestor of chimpanzees, gorillas, and humans was very much like a chimpanzee or a gorilla. Because such extensive convergence is unlikely, and because the earliest hominins are all chimpanzee-sized, the LCA is inferred to be chimpanzee-like. 

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