Declaring The Discovery Of A New Species Can Get Tricky
May was an exciting month for new discoveries that add to our knowledge of human evolution during the period around 3 million years ago. This is before the origin of the genus Homo, 2.8 million years ago, and during the time when Australopithecus afarensis (the famous "Lucy") lived in East Africa.
On May 21, details of 3.3-million-year-old stone tools from Kenya — the oldest yet discovered — were published by Sonia Harmand and her colleagues in the journal Nature. The tools consist of sharp flakes, large cores and flat anvils. According to an NPR report, "While they weren't as sophisticated as tools that have been associated with the first humans, they were definitely crafted intentionally."
That intentionally-fashioned tools predate our genus doesn't surprise me; it makes abundant evolutionary sense, given what we know of sophisticated tool-use and tool-making among animals as diverse as chimpanzees and birds.
One week later, in the same journal, Yohannes Haile-Selassie, curator and head of the physical anthropology department at the Cleveland Museum of Nature History, and his colleagues, announced a new 3.3-million-year-old species of human ancestor contemporaneous with Au.afarensis and living (as Lucy did) in the Afar region of what is now Ethiopia: Australpithecus deyiremeda. (The word "deyiremeda" means "close relative" in the local Afar language.)
Whether you find this news surprising depends, in part, on whether you're a lumper or splitter. The Wikipedia definition of these terms works well for our purposes:
"A 'lumper' is an individual who takes a gestalt view of a definition, and assigns examples broadly, assuming that differences are not as important as signature similarities. A 'splitter' is an individual who takes precise definitions, and creates new categories to classify samples that differ in key ways."
Haile-Selaisse et al.'s decision to create a new category for this East African hominin comes from the splitter camp and is bold (though certainly not unprecedented) given that the fossil material they uncovered consists entirely of jaws and teeth. These scientists are confident that there's adequate morphological variation in the maxilla (upper jaw), mandible (lower jaw) and teeth compared to earlier-described hominins to justify the new species designation. According to the Cleveland Museum of Natural History website:
"Australopithecus deyiremeda differs from Lucy's species in terms of the shape and size of its thick-enameled teeth and the robust architecture of its lower jaws. The anterior teeth are also relatively small indicating that it probably had a different diet."
Ian Tattersall, curator emeritus in the division of anthropology at the American Museum of Natural History and author of the new book The Strange Tale of the Rickety Cossack and Other Cautionary Tales from Human Evolution, agrees that we are dealing with a new species. When I asked Tattersall about the new fossil material, he responded by email on Monday in this way:
"I have only seen small publicity photos, but the new species seems entirely reasonable. In fact, my book is all about the long struggle to get paleoanthropologists to recognize diversity in human evolution, and this adds grist to the mill.
The morphological distinctiveness of the new fossils sets them apart from anything else known in this time range — or as far as I can tell any other. This diversity brings human evolution even more into line with that of other taxa, and yet farther away from the dogma of hominid linearity that has reigned since 1950."
It's certainly true that linearity, as a descriptive term for what happened during human evolution, is now a discarded concept: At least two other hominins (Kenyanthropus platyops and Australopithecus bahrelghazali) are thought to have lived in Africa during this same time period.
Still, it's far from easy for paleoanthropologists to know when it's better to carve up continuous variation in morphological traits by naming a new species and when to embrace, instead, the idea of variation within a species.
University of South Carolina anthropologist Andrew White considers some of the challenges surrounding the species concept in a blog post published May 29. White first clarifies that, among various ways to define what a species might be, in the paleoanthropological context the biological species concept is most relevant.
"A biological species is reproductively isolated from all other biological species. The biological species concept is perhaps the one most frequently applied in biology, especially to living populations of plants and animals.
I think a 'biological species' is also what most people, paleoanthropologists included, mean when they talk about "species" of hominids....
Because reproductive isolation is the entire basis of the biological species concept, individuals in a biological species (by definition) could not and did not interbreed with any of their contemporaries outside their own species."
An assertion that Au. deyiremeda is a new species equates, then, to an assertion that it could not have interbred with other hominins living nearby at the time. For White, this causes concern:
"First, I think it's impossible to operationalize consistently and objectively (how can you determine if the populations represented by fossil individuals were capable of inter-breeding?). So, I don't trust that 'species' that are equivalent in taxonomic terms reflect populations that are equivalent in evolutionary terms.
Second, I think naming lots of 'species' short-circuits our study of what variability in the fossil record means by erring on the side of attributing it to species-level differences. Under the biological species concept, species are non-overlapping, nominal categories, and speciation is a one-way street. By calling a fossil hominid a new biological species, you are making a statement about the nature of the relationship between that fossil and all other fossil hominids. Given the sparse nature of the fossil record, I don't think that we can really make those kinds of statements with much confidence."
White's two points resonate with me, especially the idea that naming a new species means that scientists automatically make probably premature statements about relationships among hominins.
Further, naming a new species brings more global, splashy media attention (and possibly more grant dollars, though I know of no way to test that particular hypothesis) compared to the weaker response won by a description of newly-understood intraspecific variation. This last point may seem cynical, and I don't mean to suggest it's a primary motivator in the case of Haile-Selassie et al. Yet, I do think it's a reality of the ways in which science discoveries are rewarded (or not).
Haile-Selassie et al.'s naming of the new australopithecine species leads them to emphasize in their Nature article the "incontrovertible evidence" showing multiple hominin species existed in East Africa during a crucial time period in our past. Taxonomic debates aside, the evidence is indeed rock-solid that before we became human, our hominin ancestors were behaviorally and cognitively complex primates. Their evolutionary story is our story, too.
Barbara J. King, an anthropology professor at the College of William and Mary, often writes about human evolution, primate behavior and the cognition and emotion of animals. Barbara's most recent book on animals was released in paperback in April. You can keep up with what she is thinking on Twitter: @bjkingape.