Why the Homo Naledi Discovery May Not Be Quite What it Seems

Human evolution: the skeletons in the closet

Homo naledi discovery
Fossils of a newly discovered ancient species, named "Homo naledi", are pictured during their unveiling outside Johannesburg September 10, 2015.Siphiwe Sibeko/Reuters

Dr. Jeffrey Schwartz PhD is professor of biological anthropology at the University of Pittsburgh in Pittsburgh, Pennsylvania.

In this year alone, the media has been treated to announcements of firsts in human evolution: the earliest stone tools [~3.3 million years (ma), Kenya], the earliest member of our genus, Homo (~2.88 ma, Ethiopia), and now, the ancestor of all members of this genus (no date, South Africa), which has been dubbed Homo naledi.

The stone tools are significant because one attribute human paleontologists (paleoanthropologists) have used for allocating a specimen to Homo is tool making. Indeed, in 1964, this was a cornerstone for Louis Leakey (of Olduvai Gorge, Tanzania fame) and colleagues creating Homo habilis, which they claimed was the earliest member of the genus. Even today, paleoanthropologists believe that Homo habilis transformed smoothly into Homo erectus, which gave rise to Homo sapiens.

Because these ancient tools are not associated with human fossils, they should provoke a change in the "man-the-tool-maker" mindset: Perhaps many other kinds of hominids (= humans and their fossil relatives) made tools. I like this, because the notion "only members of Homo make tools" has struck me as elitist. Although the quest for the earliest member of Homo continues apace, one may ask: How accurate are these claims if this year's celebrity fossil can so easily be dethroned by a newly discovered specimen?

This question leads to another: How do paleoanthropologists decide if a specimen belongs to a species – whether newly or already named – and if that species is a member of Homo? As the case of Homo habilis illustrates, it is primarily by chronology, not detailed morphology. Indeed, since 1950, when the ornithologist Ernst Mayr declared that all hominid fossils belonged in Homo, paleoanthropologists have followed suit, plunking specimens into chrono-species, and then describing the often-marked differences between them as individual variation. Other paleontologist, however, would entertain the possibility that such vast differences reflect species, if not also genus, status.

One consequence of continually lumping new specimens into a few species is that each has become so bloated with morphologically disparate specimens, that one can find some specimen in a group that is somewhat similar a new find. Further, most paleoanthropologists think that some species of australopith (such as "Lucy") had morphed into the earliest Homo, within which one species gradually transformed into another. And a consequence of this mindset that transitional forms will exhibit a mosaic of features inherited from their purported ancestor, as well as features of another.

Enter the newly announced species, Homo naledi, which is claimed to be our direct ancestor because it has features of australopiths and Homo. Why is it a species of Homo? Because some specimens seem to be like us. Why australopith? Because other specimens have some of their features. Why do all belong to the same species? Because they were found in the same cave. but, the published images tell a different story.

Viewed from the side, two partial skulls are long and low, with a long gently sloping forehead that flows smoothly into the brow – nothing like us, or most specimens regarded as Homo. A third partial skull is very short and rounded, with a high-rising forehead that is distinguished from a distinct, well-defined brow by a shallow gutter – not like the other skulls, and not like us or most specimens regarded as Homo. The femur has a small head (the ball end that fits in the hip socket) that is connected to the shaft of the bone by a long neck, and, below the neck, is a "bump" of bone that points backward. These features are seen in every australopith femur. In us, and all other living primates, the head of the femur is large and the neck short, and the "bump" points inward. Further, the teeth are very similar to those from a nearby fossil site that has yielded various kinds of australopith. Even at this stage of their being publicized, the "Homo naledi" specimens reflect even greater diversity in the human fossil record than their discoverers will admit.

What to do? As I recently advocated in the journal Science, it's about time paleoanthropologists acknowledged what a taxonomic and undefinable mess the genus Homo has become, and restudy the human fossil record without preconceived notions and the historical weight of overly used names. We must start from scratch, comparing in greater detail than usual specimens in order to see how they sort out, first into groups one might call by species, and then into larger groups we may give genus names to. It may be necessary to revive genus names that had been proposed early on, but what I predict is that we will see a picture of hominid taxonomic diversity that mirrors the diversity of virtually every other animal.

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