Ardipithicus ramidus -- Apparently It Was Worth the Wait

UC-Berkeley paleoanthropologist, Tim White, had been part of some of the most important discoveries in human origin. He was with Mary Leakey and helped describe the 3.7 million year old Laetoli footprints that showed beyond doubt Australopithicines walked on two legs. He was with Don Johanson when they discovered the most famous homonin fossil, the 3.2 million year old Australopithicus afarensis fossil better known as "Lucy". Somewhat more recently he was head scientist with a group of Ethiopian scientists who were exploring the Awash region of that country. There they made a discovery that is sure to be equally important to those other ones.

Initially they found a tooth ... then a jawbone. Even at that point White realized that this was an important find. It was a homonin older than any other at the time. He wrote an initial report giving a very brief description of the fossil. This is a common practice. It does two things; (1) it alerts the field to an important find, and (2) it establishes the scientist's priority should additional specimens be found before a detailed description comes out. But the real excitement comes when the detailed description comes out. White promised that a detail description would follow. That was a little over 15 years ago. Today eleven papers were published in the journal Science giving that detailed description.

Is it normal to wait 15 between the discovery of a fossil and the detailed description of it? Absolutely not!! White has been highly criticized over the years for not publishing earlier. I was one who criticized him for it. Was White justified in holding off so long? The jury is still out on that one. As recently as six months ago I would have said absolutely not. Today after having reaad the papers and understanding the amount of work that has been going on in the meantime, I am inclined to give him the benefit of the doubt.

So what was the hold-up? White and his group continued to sift for fossils. Now the fossil, classified as an Ardipithicus ramidus, officially numbered ARA-VP-6/500, but best known by its nickname ... Ardi ... is not just some teeth and a jaw. It is a skeleton about 40% complete. That is as complete as Lucy. Not only that but White and his team have uncovered from surrounding sites remains of at least 35 other of the same species.

A fossil skeleton can tell us a lot, but not everything we would like to know. Fossils of plants, pollen, and other animals in the same strata can tell us important information about the environment the fossil species lived in. Besides the Ardipithicus specimens White and his team discovered, they cataloged 150,000 other bits of evidence that le them to determine the environment Ardi inhabited.

Finally, Ardi had particularly fragile bones for a fossil. White referred to it as "road kill". That means that much of it was in tiny fragments that needed to be reconstructed. This process is often likened to doing a jig-saw puzzle. But it is more complicated than that. Jig-saw puzzle pieces have defined shapes and only fit together in one way. With "road kill" fossils it is often hard to tell exactly where pieces fit together. Surface anatomy is not enough to tell them exactly which piece goes where. Internal anatomy, such as tracts of blood vessels in the bones, is needed.

White and his colleagues got this information by making Computerized Tomography (CT) scans. CT scans are a type of x-ray. Multiple scans are taken from a number of different angles and a computer interprets the data to make a three-dimensional picture of the bone. For the bone fragments a special CT scanner was needed. Special programs were needed to allow the three-dimensional images to be manipulated so fragments could be joined together in virtual reality before being actually glued together.

That is a huge amount of work. It took White and his colleagues 15 years, but they weren't being lazy about it.

So what is the big deal about this specimen? Well, let's take a look at Ardi:

That is the fossil.

This is an artist's reconstruction of the fossil.

Ardi is 4.4 million years old. She is one of less than 10 skeletons over 1 million years old that are that intact. She is the oldest of these skeletons, being 1.2 million years older than Lucy, the next oldest.

In the 15 years since the discovery of Ardi, there have been three other fossil homonin that are older:

(1) Ardipithicus kadabba: 5.5 million years old, also discovered by White and his colleagues. It is known from a single canine tooth.
(2) Orrorin tugenensis: 6 million years old, discovered by Bridgette Senut and Martin Pickford. It is known from a few bones (the most important one is a femur which suggests the specimen walked upright) and some molars.
(3) Sahelanthropus tchadensis: 7 million years old, discovered by a group led by French paleoanthropologist Alain Beauvilain. It consists of a skull.

White believes that all of these share characteristics similar to Ardi. He proposes that they all should be classified in the same genus, Ardipithicus. By doing this he sees human evolution proceeding by a process similar to that proposed by famed paleontologists, Nils Eldridge and Stephen Jay Gould -- punctuated equilibrium. White sees his Ardipithicines as being fundamentally different from the Australopithicines which are fundamentally different from those members of the genus Homo (that's us). So what White sees as having happened is that some environmental change led to selective pressures that led to Ardipithicus. This would be a stable genus. Environmental selective pressures would allow speciation but there was no push for dramatic changes. Then something would have happened and selective pressures would have led to a relatively rapid evolution of Australopithicus. A second stable genus would be around. Speciation would cause diversification but no dramatic changes. Then again there would be an environmental change in which environmental selective pressures would allow the rapid evolution of Homo. Thus, there would be three plateau of evolution leading up to humans.

This will be a very controversial idea. The reason is that Sahelanthropus and Orrorin aren't well enough defined for people to agree with White's conclusion that they should all be the same genus. But it is still an interesting idea.

White comes to another controversial conclusion. What was the common ancestor of humans and chimps like? While there is a rich fossil record on the track leading to humans, the fossil record leading to chimpanzees has been almost non-existent. The idea has been that since chimpanzees and gorillas are similar and gorillas branched off of the tree before humans and chimps split, the common ancestor must be chimp-like.

However, White sees Ardi as being very close to the common ancestor. And he sees an interesting mix of similarities and differences between her and chimpanzees. For instance, her back is much longer than chimps. This is a definite homonin trait.

When she walked on land she walked upright. But she has an opposable big toe. No Australopithicine or Homo has that. So she spent a large amount of time in the trees. However, her arms were not that good for swinging branch-to-branch the way chimps of today do. She climbed vertically. To go from one tree to another she probably went down to the ground.

Consistent with this the paleobotany of the region showed a lot of fossilized wood and tree pollens. This suggested that Ardi lived in a forest environment. Australopithicines and Homo lived in savannah environments. The foot and leg arrangement showed that Ardi would have been a very poor runner.

Male primates that actively competes for females have pronounced canine teeth. None of the Ardipithicus ramadis fossils that White found have enlarged canines. Chimps do. White believes that this suggests that Ardipithicus had a different social structure. Instead of competing for females, perhaps they formed pair-bonds (the way humans of today do).

So Ardipithicus had an ecletic mix of chimp-like and Homo-like traits. This is just what we would expect in a transitional species. Was Ardi in our genealogy as one of great-great-grandmothers? Who knows. There is no way to tell with any certainty. She could have been ... or she could just as easily have been a side branch that would leave no descendants. But if she was a side branch she was very close to what would have been our ancestor. Future fossil finds have the chance of showing her to be a side branch, but no fossil find can ever show her to be our direct ancestor. You can never rule out the chance that we will find a fossil as old or older that has more Homo-like characteristics, and if we do then Ardi will be relegated to side branch status. Whatever new fossil we find will be subject to the same possibilities as Ardi is now.

Thus, in paleontology a transitional fossil is not one that is confirmed to be in the chain that leads from one fossil species to another. It is one that shows characteristics of both putative species. That is what makes Ardi an excellent transitional species. It, along with the multitude of other fossil homonin, are excellent evidence for evolution.