Note: I’m not an expert on human evolution, so this post is largely derived from some catch-up reading I had to do, and there may be some errors. Feel free to comment or correct me in the comments. I use the subspecies designation H. sapiens sapiens as a synonym for “modern H. sapiens” and to distinguish them from Neanderthals, which on reproductive grounds (see below) I consider a subspecies: H. sapiens neanderthalensis.
Hominins migrated out of Africa several times, including Homo erectus, whose members left Africa about 1.8 million years ago and spread throughout Eurasia, making it as far as to what are now China, Vietnam, and India. But for reasons unknown that species went completely extinct about 140,000 years ago, and contributed nothing to the modern human genome.
The extant species, our own Homo sapiens sapiens, evolved in Africa but also left the continent—several times. The most recent exodus was about 60,000-90,000 years ago, and descendants of those migrants include all living non-African populations of our species (we know this from genetic data). However, most modern humans also carry a bit of genome from earlier migrations: most notably that of the Neanderthals, whose ancestors probably left Africa about 300,000 years ago and went extinct between 40,000 and 25,000 years ago. Before they died out, though, they certainly interbred to some extent with the lineage that produced modern humans, so that many of us carry a handful of Neanderthal genes. (As I said at the top, I consider Neanderthals a subspecies of Homo sapiens—H. sapiens neanderthalensis—because they produced viable and fertile hybrids with “modern” H. sapiens, which I call H. sapiens sapiens to simplify matters. Some anthropologists, however, consider Neanderthals a full species: H. neanderthalensis.)
The characteristics of H. sapiens sapiens, compared to other hominins, include, as a summary in Science describes (“When did modern humans leave Africa?“; reference and link below), “a globular braincase, brow ridges that are divided into central and side portions, a flat and retracted midface, a chin on the lower jaw, and a narrow pelvis.” Hominins with these modern features, probably having split off from the H. erectus lineage, first appear roughly 300,000 years ago in Africa. There followed several bouts of migration out of Africa, producing groups like the Denisovans and Neanderthals, but until now H. sapiens sapiens fossils found outside Africa date from 120,000-90,000 years ago—in Israel. These emigrants probably went extinct, too, so though they are considered members of H. sapiens sapiens, they were a group that didn’t survive.
Now a new paper in Science by Israel Hershkovitz et al. (and there are a lot of “al.”s; see reference below with free access and full pdf here) pushes back the date of H. sapiens emigrants a long way—to roughly 180,000 years ago. These emigrants, too, apparently went extinct (we don’t know why—perhaps their population was too small?), and left no genetic contribution to the modern human genome. But the paper suggests that there were excursions of H. sapiens sapiens to lands outside Africa even longer ago than we thought.
The new data come from a single jawbone collected in Misliya cave on Mount Carmel in Israel. Here’s the collecting site from the Science paper (all captions from that paper):
What the authors found was the left side of the left upper jaw, including all eight teeth, some of the cheekbone and the roof of the mouth, and a bit of the nasal cavity (left in photo below). The specimen was dated in several ways, including uranium/thorium dating of the dentine and sediments adhering to the jawbone, uranium and combined uranium/electron-spin data on the enamel, and thermoluminescence on burnt stone tools associated with the fossils. The dating ranges are shown to the right:
The dates are pretty concordant except for the U-series on the dentine, which gives an age of about 70,000 years, not different from the most recent migration out of Africa that led to all modern extra-African H. sapiens sapiens populations. That outlier bothers me, but the authors, combining the data, come up with an age of the specimen between 177,000 and 194,000 years (dark band in the figure to the right above). This being above my pay grade, I’ll take their word that the youngest date of 70,000 years is wrong.
How do we know this is “modern” H. sapiens? The authors did morphological analysis of the teeth and show that they fall well within the boundaries of the teeth of that group. Here’s a principal component analysis of the crown shape of one molar from the specimen. (This analysis combines several features of morphology into two main axes that capture most of the variation among specimens.) In the figure below, the new molar is “Misliya1” on the left, grouping nicely with the gray diamonds of modern H. sapiens and shaped differently from the teeth of Neanderthals (black diamonds), early modern humans (presumably from Africa; greenish-yellow x’s), early and middle Pleistocene humans from Europe, which aren’t H. sapiens but in the genus Homo (purple squares), and specimens from Africa of that same age (burgundy + sign), and Middle Pleistocene Asian Homo (probably H. erectus; blue triangles). If you want the exact locations, check the paper’s supplementary data.
Granted, it’s only one molar, but the age is pretty compelling, and it does fall out with modern H. sapiens. Other data I haven’t shown, on the maxilla (jawbone) shape as well as features of other teeth, show that these, too, group with recent modern humans and not with earlier non-sapiens Homo.
Further, the tools found in the same stratum and dated with thermal methods show what is called “Levallois technology,” which is explained and animated by Wikipedia:
A striking platform is formed at one end and then the core’s edges are trimmed by flaking off pieces around the outline of the intended lithic flake. This creates a domed shape on the side of the core, known as a tortoise core, as the various scars and rounded form are reminiscent of a tortoise’s shell. When the striking platform is finally hit, a lithic flake separates from the lithic core with a distinctive plano-convex profile and with all of its edges sharpened by the earlier trimming work.
This way of making cutting tools is also seen in African specimens of H. sapiens sapiens dated about the same time, further supporting the notion that the Israel specimen came from modern H. sapiens and does go back about 200,000 years.
THE UPSHOT: This finding does not “revolutionize the story of human evolution”: after all, if these specimens are about 180,000 years old, they’re from a population of our species that went extinct without leaving descendants. But what it does show is that modern H. sapiens sapiens—members of our own subspecies—left Africa considerably earlier than we thought. And it shows that there were several migrations of H. sapiens sapiens out of Africa at different times. All modern “out of Africa” populations, however, do descend from the most recent exodus, roughly 60,000-90,000 years ago. The story is complex, and also involves mating of “modern” H. sapiens with the H. sapiens neanderthalensis lineage—both in and out of Africa.
More surprises surely await, as hominin fossils are rare, and each one can potentially tell us something amazing. As Steve Gould once said, every time he taught human evolution he threw away all of his previous notes and completely rewrote the course.
Here’s a figure from the Stringer and Galway-Witham “news and views” piece showing the different migrations of modern humans, with an even older one within Africa:
Hershkovitz, I., G. W. Weber, R. Quam, M. Duval, R. Grün, L. Kinsley, A. Ayalon, M. Bar-Matthews, H. Valladas, N. Mercier, J. L. Arsuaga, M. Martinón-Torres, J. M. Bermúdez de Castro, C. Fornai, L. Martín-Francés, R. Sarig, H. May, V. A. Krenn, V. Slon, L. Rodríguez, R. García, C. Lorenzo, J. M. Carretero, A. Frumkin, R. Shahack-Gross, D. E. Bar-Yosef Mayer, Y. Cui, X. Wu, N. Peled, I. Groman-Yaroslavski, L. Weissbrod, R. Yeshurun, A. Tsatskin, Y. Zaidner, and M. Weinstein-Evron. 2018. The earliest modern humans outside Africa. Science 359:456-459.
Stringer, C. and J. Galway-Witham. When did modern humans leave Africa? Science 359:389-390.