Recently we saw that a new study found the supposed human ancestor Sahelanthropus tchadensis had a chimp-like quadruped body plan. It therefore should not be considered a human ancestor. The hominin fossil Ardipithecus ramidus, or “Ardi,” has been going through a similar evolution.
Initially, Ardi was widely called the “oldest human ancestor,” due to its supposed skeletal traits that indicated an early bipedal (upright walking) species. Lead researcher Tim White even called Ardi the “Rosetta stone for understanding bipedalism.” But after Ardi was officially announced, other papers strongly challenged the claim that Ardi was bipedal. One article in Science commented that “All of the Ar. ramidus bipedal characters cited also serve the mechanical requisites of quadrupedality.” Another review in Nature argued strongly that “the claim that Ardipithecus ramidus was a facultative terrestrial biped is vitiated because it is based on highly speculative inferences about the presence of lumbar lordosis and on relatively few features of the pelvis and foot.”
More Strikes Against Bipedality
Now, according to a new study in the journal Science Advances, “Ardipithecus hand provides evidence that humans and chimpanzees evolved from an ancestor with suspensory adaptations,” the hand of Ardipithecus ramidus was suited for climbing and swinging in trees, and possibly also for knuckle-walking. Although the authors state that some of Ardi’s other traits may indicate upright walking (a conclusion that is disputed—see the quotes above), these are more strikes against Ardi’s bipedality. The new study states (internal citations removed):
[T]he hand of the 4.4-million-year-old hominin Ardipithecus ramidus purportedly provides evidence that the hominin hand was derived from a more generalized form. Here, we use morphometric and phylogenetic comparative methods to show that Ardipithecus retains suspensory adapted hand morphologies shared with chimpanzees and bonobos.…
Our results show that, despite subtle differences from modern suspensory apes (e.g., decreased MC5 length), the Ar. ramidus hand (ARA-VP-6/500) displays morphometric affinities with great apes and shared a selective regime with modern chimpanzees and bonobos.…
We interpret the shared aspects of nonpollical metacarpal and phalangeal morphology of Ar. ramidus, Pan, and, by estimation, the Homo–Pan LCA to reflect the use of below-branch, suspensory posture and locomotion as part of a varied positional repertoire including arboreality, vertical climbing, and, among the latter two, terrestrial quadrupedalism.…
Overall, our analysis demonstrates that the hand morphology of Ar. ramidus is more closely aligned with chimpanzees and bonobos than generalized quadrupeds, which supports the hypothesis that hominins evolved from an ancestor with a positional repertoire including suspension, vertical climbing, and, possibly, knuckle walking.
Note that they propose that Ardi had a chimp-like mode of locomotion: “suspension, vertical climbing, and, possibly, knuckle walking.” They don’t definitely say that Ardi wasn’t a biped, but they note that these traits do not indicate that Ardi was bipedal, like humans.
The magazine The Scientist has a story on the study, explaining that Ardi’s hand morphology is much closer to chimps than to humans or the australopithecines (which are thought to lie on the human branch of the tree):
The researchers integrated 26 measurements from the 2009 Ardipithecus data, including the length of finger bones and joint dimensions of the knuckles, into a statistical model that also included corresponding measurements from other primate species, both living and extinct. They used the model to group species with similar hand morphology. Ardipithecus clustered with orangutans, chimpanzees, spider monkeys, and gibbons — all of which grasp tree branches as they climb vertically and swing through the trees. Ardipithecus hands did not cluster with those of modern humans, Australopithecus — the genus that includes Lucy and is believed to be a direct human ancestor — gorillas, and baboons, all of which climb over the tops of tree branches.
The authors also performed an analysis of curvature in hand bones, which correlates with how animals move, across all the species. The curvature of the Ardipithecus hand bones again pointed to similarities with orangutans, chimpanzees, gibbons, and spider monkeys and differences from people and gorillas.
Not a Good Candidate
If Ardi’s hand is more similar to chimpanzees than to humans, indicating Ardi has a chimplike mode of locomotion (suspension in trees, vertical climbing, and knuckle walking), one would think that this would suggest that Ardi is more closely related to chimps than to humans—probably on the chimp line and not a good candidate for a human ancestor. But we’re dealing with cladistics, a method of classification thoroughly steeped in evolutionary assumptions. For example, cladistics analyses depend on assumptions about what the most recent common ancestor (MRCA) of a group looked like, which determines “what direction” traits are evolving in (their “polarity”), which determines when traits are being gained or lost in a clade, which dictates which traits can be used when postulating evolutionary relationships. Thus, “all cladistic methods for inferring polarity depend on a priori assumptions of relationships” (Bryant, 2001), and “Errors in determining polarity of characters therefore clearly can produce errors in inference of phylogeny” (Hickman et al., 2008). The very concept of character polarity “requires the assumption of evolution” (Kluge, 1983, quoted in Hull, 2010).
In this case, under cladistics, the paper’s authors believe that Ardi can be retained as a human ancestor—despite its similarities to chimps—if we make lots of evolutionary assumptions about what the common ancestor of chimps and humans looked like, such as assuming that the MRCA of chimps and humans had chimplike hands and chimplike modes locomotion. Under these assumptions, such traits become symplesiomorphic “retentions,” which means these chimplike traits are essentially ignored when doing a phylogenetic analysis about whether Ardi is more closely related to chimps or humans. This controversial logic alludes to the classic criticism of cladistics, where crocodiles and birds are said to be closer than crocodiles and snakes, even though birds aren’t reptiles and crocodiles and snakes both are. The point is this: lots of evolutionary assumptions are required to lead one to conclude that just because Ardi has many traits—including a mode of locomotion—that are more similar to chimps, it’s actually a much closer evolutionary relative to humans than to chimps.
But outside the context of evolutionary assumptions about common ancestry, this kind of evidence might point away from Ardi being closer to humans than to chimps, not towards it. Only one scientist quoted at the end of the article seemed to appreciate that Ardi might be as close to humans as some are saying:
Madelaine Böhme, a paleontologist at the University of Tübingen in Germany who was not involved in the work, notes that while the authors “are able to show the suspensory or chimp-like anatomy of Ardi,” it’s still unclear if Ardipithecus is an ancestor of Australopithecus, she says. “It’s an assumption, and this assumption is poorly supported.”
Indeed, the assumption that Ardi was a human ancestor seems more “poorly supported” all the time.
This post was updated.