Both Nature and Science Daily are touting a “new human ancestor” that was discovered in Ethiopia. As always, the dutiful science news media repeat the claim, with near-identical headlines at CBS News (“Newfound human ancestor may have lived alongside Lucy”), Washington Post (“Scientists discover a new human ancestor that roamed with ‘Lucy'”), Boston Herald (“Study: Ethiopian fossils indicate new forerunner of humans”), The Verge (“New human ancestor shared its turf with ‘Lucy'”), and elsewhere.
What, exactly, was found? Nothing more than teeth and jaw fragments. (Lest anyone misunderstand, the title of this post is facetious. While that should be obvious, I find that many things that should be obvious in the evolution debate turn out not to be so.) In all seriousness, here is the totality of all we know about this purported new species, Australopithecus deyiremeda:
Reprinted by permission from Macmillan Publishers Ltd: Nature, Yohannes Haile-Selassie, Luis Gibert, Stephanie M. Melillo, Timothy M. Ryan, Mulugeta Alene, Alan Deino, Naomi E. Levin, Gary Scott & Beverly Z. Saylor, “New species from Ethiopia further expands Middle Pliocene hominin diversity,” 521:483-488 (May 28, 2015).
It’s on the basis of this sparse evidence that we are presented with a “new human ancestor.” Here is Science Daily‘s evolutionary interpretation:
“The new species is yet another confirmation that Lucy’s species, Australopithecus afarensis, was not the only potential human ancestor species that roamed in what is now the Afar region of Ethiopia during the middle Pliocene,” said lead author and Woranso-Mille project team leader Dr. Yohannes Haile-Selassie, curator of physical anthropology at The Cleveland Museum of Natural History. “Current fossil evidence from the Woranso-Mille study area clearly shows that there were at least two, if not three, early human species living at the same time and in close geographic proximity.”
Some scientists aren’t so sure that we have enough evidence to understand this species’ relationship to other hominins:
Fred Spoor, a palaeontologist at University College London who wrote an accompanying News and Views article on the Nature study, speculates that the two species may both have been able to thrive side-by-side because they might not have directly competed for food, shelter and territory. The distinct jaw shapes of A. deyiremeda and A. afarensis could mean that they used their teeth on different kinds of food. But with so little evidence to hand, Spoor warns against jumping to conclusions about the relationship between the two species. “We shouldn’t suddenly think they stood at the Awash River, shook hands and said, ‘What are you doing here?'”
Spoor continues in Nature:
Finding such taxonomic diversity raises the question of how multiple species could have coexisted over a long period in a stable ecosystem, particularly when they live in close geographic proximity, as seems to be the case with A. deyiremeda and A. afarensis. Niche partitioning, involving diverse dietary preferences, foraging strategies, habitat selection and population movements, will probably be the key factor. However, establishing a concrete link between such characteristics and the morphological differences that distinguish species is often difficult, not least because the morphology may be affected by random genetic drift as much as by selection.
However, a further complication now emerges: the K. platyops paratype is a maxillary fragment that was associated with the holotype (main defining specimen) on the basis of three shared features: forward cheek bones, three-rooted premolars, and a small first-molar crown. But these three features are now also found in A. deyiremeda, and the distinctive front part of the maxilla that sets K. platyops apart is not preserved in the paratype. Hence, it cannot be excluded that it was A. deyiremeda that made an appearance at the tool site. Regardless, associating stone tools with a specific species as the toolmaker is notoriously difficult, and the increasingly rich fossil record of the middle Pliocene provides plenty of opportunity for lively debate.
And there’s the rub: there’s so little evidence here, it’s unquestionable that we’re going to see a “lively debate” over whether this new species is a “new human ancestor.” No wonder the lead author was quoted saying, “Some of our colleagues are going to be skeptical about this new species, which is not unusual.” But in that case, how do we know it was a human ancestor? As we discuss in Discovering Intelligent Design, this field is plagued by its paucity of data:
The origin of humans — studied by paleoanthropologists — is perhaps the most hotly debated evolutionary field. An article in the journal Science explained how biases and bitter disputes between researchers permeate the study of human evolution:
The field of paleoanthropology naturally excites interest because of our own interest in our origins. And, because conclusions of emotional significance to many must be drawn from extremely paltry evidence, it is often difficult to separate the personal from the scientific disputes raging in the field.
Raging disputes? Some might wonder how scientists can look at the same fossils and develop notably different conclusions. It’s because much of paleoanthropology is not based on objective criteria, but on subjective interpretations of scant fossil evidence.
The one point that most paleoanthropologists seem to agree upon is their belief that humans evolved from ape-like ancestors. Beyond that, the details get very sketchy and inconsistent.
This new species is a reminder of how terribly confused the hominin tree is.
As the original Nature paper explains, the jaw and teeth in question have characteristics similar to those of members of the genera Paranthropus and Homo — hominins that come millions of years later:
The morphology of Au. deyiremeda also reinforces concerns related to dentognathic (that is, jaws and teeth) homoplasy in Plio-Pleistocene hominins, and shows that some dentognathic features traditionally associated with Paranthropus and Homo appeared in the fossil record earlier than previously thought. … The taxonomy and phylogenetic relationships among early hominins are becoming more complicated as new taxa are added to the Pliocene fossil record and the temporal range and systematics of early Homo are reconsidered. A phylogenetic analysis of the preserved morphological features of Au. deyiremeda fails to produce a single most-parsimonious cladogram. However, the results are fairly consistent in placing this species as a sister taxon to a clade including Au. africanus, Paranthropus and Homo. There are many alternative phylogenies consistent with this topology, but the fact that Au. deyiremeda is positioned outside the Paranthropus and Homo clade implies that some features associated with one or both of these taxa are homoplastic. Therefore, the addition of Au. deyiremeda reinforces questions about dentognathic homoplasy in later Pliocene and early Pleistocene hominins.
Let me try to translate: This new middle-Pliocene hominin species Au. deyiremeda is thought to have various tooth and jaw characteristics similar to much later hominins like Paranthropus or Homo. From an evolutionary perspective, that’s strange, because it’s more similar to those later hominins than it is to middle hominins like Au. afarensis (Lucy’s species) that are supposed to be more closely related. Hence the reference above to “dentognathic homoplasy,” which just means the species have tooth and jaw characteristics that don’t fit the standard evolutionary tree. What’s the upshot? If this new species is a human ancestor, its more-Homo-than-Lucy-like jaws and teeth would probably mean that Lucy is not a human ancestor.
Indeed, at least one news story suggests just that: Au. deyiremeda has kicked Lucy out of our ancestral lineage:
Was “Lucy,” as the hominid was called, the direct ancestor of Homo sapiens? Was she “The Mother of Mankind,” as some headlines claimed?
Over the years, the dramatic assertion has come under attack by doubters, who point to ancient yet inconclusive finds in Kenya and Chad.
But a new fossil, reported on Wednesday, may have dealt Lucy’s claimed status an irreversible blow.
Another species of hominid lived at the same time and in the same Afar region of Ethiopia, according to the paper, published in the journal Nature.
Named Australopithecus deyiremeda, the hominid and Lucy are probably only part of a wider group of candidates for being our direct forerunners, the finders said.
The picture is murky, in short, and the evolutionary account of human origins is not getting any clearer. The subject is fascinating, and important. For more information, be sure to check out the Discovering Intelligent Design curriculum, which now includes a new free online companion. The latter features self-grading quizzes and video introductions (by me) to each section of the textbook. Enjoy!