Fossil Hominids: Ardi

ARA-VP-6/500, "Ardi", Ardipithecus ramidus

Discovered by a team led by Tim White in 1994 at Aramis in Ethiopia (White et al. 2009; Gibbons 2009). Its age is about 4.4 million years. Ardi is a spectacularly complete fossil. About 45% of her skeleton was found, including most of the skull, pelvis, hands and feet, and many limb bones. She was about 120 cm (3'11") tall and weighed about 50 kg (110 lbs).

The first ramidus fossils were found in the early 1990s, and published in a paper in Nature in 1994 (White, Suwa and Asfaw 1994), where they were assigned to a new species, Australopithecus ramidus. The bones consisted only of a few dental fragments and some arm bones. About 8 months later, a correction was published in Nature (White et al. 1995) reassigning the bones to a new genus, Ardipithecus, and mentioning that in late 1994 they had discovered a new lower jaw and partial skeleton of ramidus. Clearly, these new finds had strengthened their earlier suspicion that ramidus might not belong in the genus Australopithecus. In later articles, they revealed that the partial skeleton was extremely fragile, that excavation of it was proceeding extremely slowly, and that it might be a while before it was fully extracted and analyzed. That turned out to be a understatement - it has taken nearly 15 years to extract and analyze the skeleton.

The ramidus team has put the time to good use, however. As well as excavating and restoring the Ardi skeleton, they have discovered 110 ramidus fossils from at least 35 individuals. They also comprehensively searched the site where the skeleton was found collecting absolutely everything there, eventually ending up with over 150,000 specimens including pollen, plants, wood, insects, snails, birds, and animals. This huge haul allowed them to accurately reconstruct the environmental conditions of 4.4 million years ago. As was originally suggested when the first ramidus fossils were published 15 years ago, their environment was a woodland setting with small patches of forest, which seems to put paid to the earlier popular idea that human bipedality was somehow linked to the savannah environment inhabited by some of the australopithecines.

Usually significant new fossils have a paper published describing them, or perhaps two, one devoted to the fossil and one to its geological setting. In this case, the ramidus team, in a scientific tour de force, have released 11 papers simultaneously in the journal Science. These cover the fossil, various specialized aspects of its anatomy, the geology, the environment in which it lived, and its implications for the human evolution.

Ar. ramidus is considerably more primitive than the australopithecines. The skull and brain size are very small, comparable to a chimpanzee. The teeth fossils show that ramidus was omnivorous, unlike chimps which are adapted to a diet of mostly fruit, and australopithecines which were adapted to heavy chewing on abrasive foods. ramidus also has greatly reduced canine teeth in the males, compared to apes. This is important because in apes canine teeth are important weapons against other males in the social group, so the diminished canines probably indicate a significant change in the social dynamics of ramidus.

The leg and pelvis bones show only imperfect adaptation to bipedalism, compared to australopithecines. The skull is quite similar to the skull of Sahelanthropus tchadensis, known from a 6 to 7 million-year-old fossil skull nicknamed Toumai which was discovered in Chad in 2001. This raises the possibility that S. tchadensis may end up being reassigned to the genus Ardipithecus.

Ar. ramidus was quadrupedal in the trees, walking along branches using the palms of their hands, as many monkeys do. (i.e. they weren't hanging from branches or climbing with a vertical torso like modern apes do). It is also claimed by its discoverers that ramidus was bipedal on the ground, though not nearly as well adapted to it as humans are. (This particular claim is being treated cautiously so far; a few important scientists have expressed reservations about it.) The foot of ramidus had a widely divergent big toe for grasping while climbing (like chimps), but lacks the extreme feet flexibility of chimps which allows them to mold their feet around objects. ramidus also has none of the anatomical specializations of chimps for knucklewalking when on the ground.

The Ar. ramidus pelvis was very ape-like in the lower pelvis, but had changes in the upper pelvis which made it an effective upright walker. It is more primitive than the pelvis of australopithecines, which were almost fully adapted to bipedality.

What is all the talk in headlines about Ar. ramidus refuting the idea of a missing link? Although it is true that no-one claims humans evolved from chimps, there is still a general perception that the last common ancestor of humans and chimps was, if not a chimp, at least somewhat chimp-like (hence the idea of a 'missing link' between chimps and humans). This ramidus skeleton disproves that, according to the discoverers. As we consider modern humans, H. erectus, H. habilis, australopithecines and ramidus, their skeletons become less and less like modern humans, but they are not becoming more and more chimp-like. Au. ramidus lacks almost all of the advanced specializations of modern chimps such as knuckle-walking and brachiation. The hands and feet of ramidus, and possibly our common ancestor with chimps, seem to have been relatively unspecialized, and in some ways more similar to our hands and feet than to those of chimps.

Ardipithecus and creationists

Creationists have, of course, commented on this new fossil (e.g. Answers in Genesis, the ICR, Discovery Institute, Creation Ministries). But, exciting as this find is for scientists, it won't have much significance in the creation/evolution debate. For creationists, this is unquestionably an ape. It is, after all, more primitive than other australopithecine fossils that they already classify as apes, so the hominid features in pelvis, teeth, locomotion aren't going to concern them at all.

There was one quote from the Institute for Creation Research which I thought interesting:

There is still no solid evidence to support the fanciful idea that humans evolved from primates. This stands to reason, since mankind was specially created from the beginning.

That sums up the usual creationist attitude: evidence is irrelevant. No fossil can possibly be evidence for human evolution, no matter what it looks like, because they already know that evolution didn't happen.

References

Gibbons, A. (2009): A new kind of ancestor: Ardipithecus unveiled. Nature, 326:36-40.

How Humanlike Was "Ardi"? Katherine Harmon, Scientific American

White T.D., Asfaw B, Beyene Y., Haile-Selassie Y., Lovejoy C.O., Suwa G., WoldeGabriel G. (2009): Australopithecus ramidus and the paleobiology of early hominids. Science, 326:75-86.

White T.D., Suwa G., and Asfaw B. (1994): Australopithecus ramidus, a new species of early hominid from Aramis, Ethiopia. Nature, 371:306-12.

White T.D., Suwa G., and Asfaw B. (1995): Australopithecus ramidus, a new species of early hominid from Aramis, Ethiopia. Nature, 371:306-12.

Links

Science's Ardipithecus page

Ardipithecus: We Meet At Last, by Carl Zimmer

Oldest skeleton of human ancestor found, by James Shreeve

Bye-bye, missing link: Meet "Ardi", by Answers in Genesis

A New Evolutionary Link? Australopithecus sediba Has All the Wrong Signs, by Brian Thomas, Institute for Creation Research.

Bones of "Ardi", New Human Evolution Fossil, "Crushed Nearly to Smithereens", by Casey Luskin, Discovery Institute

Ardipithecus again, by Creation Ministries

This page is part of the Fossil Hominids FAQ at the talk.origins Archive.