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Becoming Human

The last common ancestor of humans and chimps probably wasn’t much like either

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12:00am, September 30, 2013
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Picture it: an African forest 7 million years ago. An evolutionary split is under way. One species of ape is about to give rise to two distinct lineages; one leading to humans, the other to chimpanzees. What does this last common ancestor of humans and their closest living relatives look like?

For years, many researchers have just imagined a chimpanzee.

At first glance, that seems sensible. Even though chimpanzees are genetically closer to humans than they are to the other great apes, chimps appear to have much more in common with gorillas and orangutans than with humans. These apes all look so similar and primitive: shaggy beasts with long arms and handlike feet for climbing and swinging through trees. Humans are the oddballs in this group. With naked bodies, nimble hands, a two-legged stance and, of course, supreme intellect, it seems logical that hominids have changed much more over the last 7 million years than chimps and their ancestors. This kind of thinking has led some scientists to view chimpanzees as a kind of baseline from which hominid anatomy and behavior evolved.

But over the last few decades, anthropologists have realized that this view of evolution is too simplistic and human-centric — and insulting to chimpanzees. In reality, our closest living cousins are not frozen in time; the chimp lineage has undergone its own evolution over the last several million years.

One source of flawed thinking about the human-chimp ancestor may be the sparse fossil record of chimps and their predecessors. Scientists have little tangible evidence to track chimp evolution, so it’s easy to imagine that their lineage hasn’t changed much. But there are plenty of signs that these apes aren’t stuck in a time warp.

Some of those signs are written in DNA. In 2007, biologists reported that chimpanzees have more genes that appear to have been changed by natural selection than humans do (233 versus 154 out of nearly 14,000 shared genes).

Behaviorally speaking, chimps are also unlikely to be carbon copies of their last common ancestor with humans, a fact demonstrated by Czech researchers in August. The team used a family tree of apes and monkeys and 65 characteristics related to development, ecology and mating and social behavior to reconstruct ancestors for various branches in the tree. The findings suggest that both humans and chimpanzees evolved a plethora of unique traits since separating from their common ancestor, Pavel Duda and Jan Zrzavý of the Czech Republic’s University of South Bohemia conclude in the Journal of Human Evolution. (Gorillas, by comparison, may be quite primitive, having kept many attributes of the common ancestor of great apes and humans that lived roughly 15 million years ago.)

With their analysis, Duda and Zrzavý paint a picture of the lifestyle of the human-chimp ancestor. They speculate that the ape lived in groups where one male bred with several females and he provided some care and protection to his progeny. In contrast, modern chimps are more promiscuous, living in large communities where many males mate with many females and vice versa. Humans are considered monogamous, although our mating behavior does vary around the world. 

Other evidence hints that the body plan of chimpanzees has also changed dramatically since the human-chimp split, according to anthropologists who unearthed and analyzed what they claim is the closest thing anybody has ever seen to a fossil of a human-chimp ancestor. Discovered in Ethiopia, the 4.4-million-year-old Ardipithecus ramidus is the earliest hominid for which scientists have found a nearly complete skeleton. In 2009, researchers unveiled an assessment of the species’ bones. Based on what they saw, they gave the human-chimp ancestor a complete makeover.

Before then, many scientists thought hominids descended from a tree-swinging ape that walked on its knuckles when it visited the forest floor, just like modern chimps and gorillas. But aspects of A. ramidus’ hands, feet, spine, hips and limbs indicate that the species must have instead originated from an ape that was quite monkeylike. Rather than hanging from tree limbs, the ancestors of hominids (and therefore chimps) probably walked on all fours on the tops of tree branches, C. Owen Lovejoy of Kent State University in Ohio and colleagues proposed.

That assessment implies that chimpanzees and gorillas independently evolved their tree-swinging ways. Lovejoy’s team suggests this kind of arboreal behavior arose in both apes because they increasingly relied on fruits and leaves in the treetops while human ancestors depended more on terrestrial foods. Eventually, as forests thinned out in Africa, knuckle-walking emerged in both gorilla and chimp lineages as a way for the tree-climbers to travel between patches of forest.

Not all anthropologists agree with this analysis of A. ramidus, or even that the species was a hominid. But the study does highlight that chimpanzees aren’t living fossils. That doesn’t mean that studying chimps won’t shed light on our evolutionary history; it just means that researchers shouldn’t think of the apes as direct portals to the past.

Citations

P. Duda and J. Zrzavý. Evolution of life history and behavior in Hominidae: Towards phylogentic reconstruction of the chimpanzee-human last common ancestor. Journal of Human Evolution. Published online August 24, 2013. doi: 10.1016/j.jhevol.2013.07.009.

W.C. McGrew. In search of the last common ancestor: new findings on wild chimpanzees. Philosophical Transactions of the Royal Society B. Vol. 365, October 27, 2010, p. 3267. doi:10.1098/rstb.2010.0067.

T. Harrison. Apes among the tangled branches of human origins. Science. Vol. 327, January 29, 2010, p. 532. doi: 10.1126/science.1184703.

C.O. Lovejoy et al. The great divides: Ardipithecus ramidus reveals the postcrania of our last common ancestors with African apes. Science. Vol. 326, October 2, 2009, p. 100. doi:10.1126/science.1175833.

M.A. Bakewell, P. Shi and J. Zhang. More genes underwent positive selection in chimpanzee evolution than in human evolution. Proceedings of the National Academy of Sciences. Vol. 104, May 1, 2007, p. 7489. doi:10.1073/pnas.0701705104.

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