Thursday, December 15, 2016

New fossil hominin trackways from famous Laetoli site

Newly discovered Laetoli prints from Masao et al. 2016

Back in July of this year, I posted a brief note about news articles from Africa that described new hominin trackways found at the Laetoli footprint site.  Yesterday, eLife (the journal that brought us the first reports of Homo naledi) published a paper by Masao and colleagues describing these new trackways.

According to the report, the new prints were discovered during a survey to estimate the impact of a new museum construction project at Laetoli.  Contrary to earlier reports, they are actually 150 meters south of the previously described tracks.  They are in the same rock layer as the original tracks (although the authors aren't entirely certain they were made at the same time), and they're going in the same direction.  The biggest news is that they are substantially larger than the previous tracks.  The researchers estimate that the creatures who made the tracks were about 165 cm tall and around 45 kg (that's about 5'5" and 99 pounds).  The largest of the track makers from the previously-discovered tracks was about 142 cm tall and 38 kg (4'8" and 84 pounds).  The researchers attribute these new tracks to Australopithecus afarensis.

Paleontologists have known for a long time that A. afarensis exhibited substantial sexual dimorphism.  That means that the males were on average larger than the females.  Very early in the study of A. afarensis, paleontologists noticed that the bones they were finding were of two different sizes.  The Lucy skeleton was probably around 3'7" in life, but the male Kadanuumuu skeleton was probably more than 5' when alive.  So if these new tracks really were made by A. afarensis, it wouldn't be really surprising that they are larger than the previously known Laetoli tracks.

What do I think about them?  My comments from July are still quite relevant:
I'm honestly not sure what to make of the original Laetoli prints.  Ten years ago, I would have said they were prints of australopiths, but since working on hominin baraminology, I'm not so sure we can tell the difference between human and australopith just from their footprints.  Certainly, habilis, sediba, and naledi humans probably had very different gaits from us modern humans (or even erectus humans).
I agree that I'm not sure we can tell human from nonhuman based only on their footprints.  Even in this case, where the footprints are significantly "older" (3.6 million radiometric years) than the oldest human fossil as recognized by baraminology (at 2.0 million radiometric years) and the hominin fossils present are all from A. afarensis, I'm still not willing to say for sure who made these prints.  The radiometric dates probably represent months or a few years in real time, and the overall spottiness of the hominin fossil record makes me wary of saying that the absence of human fossils at any one locality is the same as local absence of humans.

Whatever they are, they're an amazing reminder that valuable discoveries may still be lurking right under our noses.  Last year, we got Homo naledi from the well-explored Rising Star Cave, and this year, new hominin footprints from the famous Laetoli site.  What could be next?

Masao et al. 2016. New footprints from Laetoli (Tanzania) provide evidence for marked body size variation in early hominins. eLife 5:e19568.

Get free scans of the prints from Morphosource!

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