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Stop Me if You’ve Heard This One Before: Much Ado over “Oldest Human Fossil”

In 2001, the New York Times ran the headline “Fossils May Be Earliest Human Link.” In 2009, Discovery Channel announced, “‘Ardi,’ Oldest Human Ancestor, Unveiled,” and Associated Press’s story read, “World’s oldest humanlinked skeleton found.”

Here we go again.

Headlines over the last week have included “‘First human’ discovered in Ethiopia” (BBC), “Human Origins Just Got a Lot Older” (Slate), and “Jaw Fossil In Ethiopia Likely Oldest Ever Found In Human Line” (NPR), among many others.

As the NPR story suggests, all that paleoanthropologists have found is a couple of bone scraps from a hominid jaw. Next thing you know we’re “redrawing” the old “family tree” as National Geographic put it, since these bones are supposed to be the “oldest human fossil found.” If reports are correct, these 2.8-million-year-old fossils are the oldest specimens of Homo by about 400,000 years. See here for a photo of the left half of the jaw.

They also found part of the right half of the mandible, which is in even worse condition and is devoid of teeth.

Now I don’t necessarily have a particular reason to doubt that these partial jaw scraps came from Homo. But I do know that other paleoanthropologists have warned that teeth and skull scraps (collectively termed craniodental bones) can be highly plastic and make for poor diagnostic features when it comes to hominid classification. As one article stated:

Recent research has cast doubt on the reliability of bones and teeth for reconstructing phylogenetic relationships among higher primate species and genera. … [R]esearchers have begun to question the reliability of bones and teeth for reconstructing phylogenetic relationships among higher primate species and genera. This skepticism is based partly on the fact that phylogenetic analyses of fossil primates have thus far yielded conflicting and weakly supported hypotheses of relationships, partly on the fact that comparisons between craniodental phylogenies and reliable molecular phylogenies have found that the former disagree with the latter, and partly on the fact that we are developing a better understanding of the processes involved in the generation of osteological and dental similarities and differences among primates. … [T]ooth morphology is prone to homoplasy and is therefore a poor guide to low-level phylogenetic relationships.

(Sally Gibbs, Mark Collard, and Bernard Wood, “Soft-tissue characters in higher primate phylogenetics,” Proceedings of the U.S. National Academy of Sciences, Vol. 97: 11130-11132 (September 26, 2000).)

Another paper similarly reported: “Little confidence can be placed in phylogenies generated solely from higher primate craniodental evidence.”

Assuming it is Homo, what’s driving the hype this story has received is not simply the prospect that it’s the “oldest human fossil” but, instead, fantastical suggestions that somehow the specimen represents a transitional form between the australopithecines and later Homo. Yes, that’s right, this tiny scrap of jaw is being called a “missing link” — a Forbes tech writer even ran the headline “Human Missing Link – Fossil Found.”

Because this fossil comes from a period of the fossil record when we have very few bones, and because it was found in Ethiopia about ten miles from the site where Lucy (Australopithecus afarensis) was discovered, some are suggesting the jaw shows that early members of Homo that descended directly from Lucy’s species. One paleoanthropologist told CNN that these meager jaw scraps represent “an excellent case of a transitional fossil in a critical time period in human evolution.”

Again, even though all they found was a tiny scrap of a jaw, here’s how BBC News spins it:

The head of the research team told BBC News that the find gives the first insight into “the most important transitions in human evolution”.

Prof Brian Villmoare of the University of Nevada in Las Vegas said the discovery makes a clear link between an iconic 3.2 million-year-old hominin (human-like primate) discovered in the same area in 1974, called “Lucy”.

Could Lucy’s kind — which belonged to the species Australopithecus afarensis — have evolved into the very first primitive humans?

“That’s what we are arguing,” said Prof Villmoare.

National Geographic frames it this way:

The date is tantalizingly close to the last known appearance, around three million years ago, of Australopithecus afarensis, an upright-walking, small-brained species best known from the skeleton called Lucy, believed by many scientists to be the direct ancestor of our genus. The new jaw, known as LD 350-1, was found in January 2013 just a dozen miles from where Lucy was found in 1974.

But once again, bear in mind that all they found was a small scrap of jaw. Paleoanthropologist William Kimbel tries to temper the excitement by reminding us just how little evidence there is to go on:

Fossils attributed to Homo in the period two to three million years ago are exceedingly rare. Bill Kimbel, director of the Institute of Human Origins at Arizona State University, in Tempe, who co-led the analysis of the new specimen, once said that “You could put them all into a small shoe box and still have room for a good pair of shoes.”

If these fossils are indeed from Homo, then that has major implications for evolutionary models that prefer Australopithecus sediba as a transitional form leading to our genus. Au. sediba doesn’t appear for another million years after these fossils, meaning that if these jawbones really do belong to Homo, then sediba could not be ancestral to Homo. National Geographic explains:

To the extent that the new jaw underscores an East African origin for the genus Homo, it would seem to confound the argument made by other researchers that the best candidate for our genus’s immediate ancestor is a South African australopithecine, Australopithecus sediba.

The authors of the Science paper point out that the only known specimens of A. sediba are almost a million years younger than the new Homo jaw from Ethiopia that they would have had to have given rise to.

But then of course the defenders of Au. Sediba point out that this jaw bone hardly establishes anything:

“The idea that [the new jaw] makes anything else unlikely to be an ancestor is ludicrous,” says Grine. “That would pretend that the fossil record is complete. And we know it can’t be, since they just discovered something that wasn’t there before.”

And NPR reports:

Villmoare says he doesn’t yet know anything about the creature’s brain; there’s nothing from the cranium in this find. But he says its jaw and teeth clearly have human traits.

Likewise, Sci-Tech Today reports:

The new paper’s analysis is first-rate, but the fossil could reveal only a limited amount of information about the creature, said Eric Delson of Lehman College in New York.
“There’s no head, there’s no tools, and no limb bones. So we don’t know if it was walking any differently from Australopithecus afarensis,” which was Lucy’s species, he said.

Thus, it’s very hard to use this little jaw fragment of poor quality to demonstrate a clear link with just about anything. Amidst the hype, Chris Stringer of the Natural History Museum in London introduce a note of caution, and sanity: “Half a jawbone is not enough to tell just how human it was and does not provide enough evidence to suggest that it was this line that led to us.”

But that’s the point, isn’t it? This scrap of bone establishes very little. And so you have defenders of one evolutionary thesis attacking defenders of another evolutionary thesis on the grounds that the fossil record is too poor to derive any conclusions. If that’s the case, then how do we know that humans evolved from apelike species (like Lucy’s species Australopithecus afarensis) to begin with?

The answer, in case you were wondering, is that we don’t. If we had virtually no fossil evidence documenting the evolution of Homo from apelike creatures prior to this find, then arguably after this find, we still have virtually nothing.