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  • Translator: Leslie Gauthier Reviewer: Krystian Aparta

  • Human origins.

  • Who are we?

  • Where do we come from,

  • and how do we know?

  • In my field, paleoanthropology,

  • we explore human origins --

  • the \"who\" and \"where\" questions --

  • by analyzing fossils that date back thousands and even millions of years.

  • In 2015, a team of colleagues and I named a new species in the genus Homo --

  • our genus --

  • Homo naledi.

  • Let's take a step back and put that into context.

  • The last common ancestors between humans and chimps

  • date somewhere between six and eight million years.

  • The earliest hominins,

  • or earliest human ancestors,

  • evolved into a group known as the australopithecines.

  • The australopithecines evolved into the genus Homo

  • and eventually modern humans -- us.

  • With each new fossil discovery,

  • we get a little bit closer to better understanding who we are

  • and where we came from.

  • With these new fossil finds,

  • we realize we now have to make changes to this tree.

  • Until this discovery,

  • we thought we had a pretty good idea about the patterns of evolutionary change.

  • Current fossil evidence suggests

  • that the earliest populations of the genus Homo evolved in Africa

  • somewhere between two and three million years.

  • Fast-forward to approximately 300,000 years to where we see the origins

  • of the first modern humans.

  • While the fossil record between these time frames in Africa

  • is relatively sparse,

  • the fossils nonetheless demonstrated certain trends

  • from our earliest ancestors to modern humans.

  • For example, our brains were becoming larger

  • relative the rest of our body.

  • Our pelves were becoming more bowl-shaped,

  • and our hand-wrist morphology, or form,

  • suggested a change in our grip as we began to make and use stone tools

  • and spend less time in the trees.

  • These new fossils disrupt everything we thought we knew about these trends

  • and force us to change the way that we think about human evolution.

  • South Africa in general,

  • but the Cradle of Humankind in particular,

  • contains numerous sites where hundreds of thousands of fossils have been found.

  • As an undergraduate student, I fell in love with one of them ...

  • Mrs. Ples.

  • The skull of a 2.1-million-year-old early human ancestor.

  • From that point on,

  • I was determined to go to South Africa and study human evolution.

  • I first traveled there in 2003,

  • and I did get to see my beloved Mrs. Ples.

  • (Laughter)

  • But words can hardly convey my excitement

  • when I was chosen as an early career scientist

  • by Lee Berger,

  • a world-renowned paleoanthropologist,

  • to be one of the primary analysts of recently excavated unpublished fossils.

  • This treasure trove of fossils was being recovered from a new site

  • called the Dinaledi Chamber in the Rising Star cave system.

  • Species are often named based on a skull, a lower jaw,

  • or, very rarely,

  • a handful of postcranial, or below-the-neck, elements.

  • The fossils from Dinaledi were another story altogether.

  • An unprecedented approximately 1800 specimens --

  • so far --

  • have been excavated from the Rising Star system,

  • representing at least 15 individual skeletons.

  • The research team that I was invited to join

  • was tasked with describing, comparing and analyzing the fossils,

  • with the difficult goal of identifying to what species the fossils belonged.

  • We were divided up into our different areas of expertise.

  • We were divided up in different areas of the lab, too.

  • So there was \"Hand Land,\" for the fossil hand people,

  • \"Hip Heaven\" for the pelvis ...

  • I was in the \"Tooth Booth.\"

  • (Laughter)

  • And after long, intense days in the lab,

  • the different teams would meet up at night and discuss our findings,

  • still consumed by questions from our analyses.

  • It was incredible how different the interpretations were.

  • Each body part seemed to come from a different species,

  • based on what we knew from the fossil record.

  • The suite of characteristics we were seeing didn't match any known species.

  • And if we had only recovered the skull, we might have called it one thing;

  • if we had only recovered the pelvis, we might have called it another.

  • The anatomy of the skeletons didn't make sense

  • with the framework of what we thought we knew of human evolution.

  • Did it belong in the genus Homo?

  • Should it be an australopithecine?

  • Those bipedal, more apelike ancestors?

  • Or perhaps it should be its own species.

  • Ultimately, after much deliberation,

  • we decided the Rising Star specimens did indeed warrant a new species,

  • which we called \"Homo naledi.\"

  • From the head to the feet,

  • the fossils present a mosaic of primitive, or ancestral,

  • and derived or more modern-like features.

  • The skull is quite derived,

  • appearing most similar to early representatives of the genus Homo,

  • like Homo habilis and Homo erectus.

  • However, the brain is scarcely half the size of a modern human one.

  • One that is smaller than any other early Homo that has ever been found.

  • As someone who studies teeth,

  • I might argue these are the coolest fossils found at the site.

  • (Laughter)

  • The assemblage consists of 190 whole or fragmentary teeth

  • that range in age from very old to very young.

  • Like the skeletons,

  • the teeth present a mix of primitive and derived traits.

  • In modern humans,

  • the third molar is typically the smallest, while the first molar is the biggest,

  • but Homo naledi has the primitive condition

  • where the third molar is the biggest and the first molar is the smallest.

  • The anterior teeth,

  • or the incisors and canines,

  • are small for the genus Homo,

  • and the lower canine has a cuspulid on it --

  • an extra cuspule that gives it a distinct mitten-like shape

  • that it shares with some specimens of the early human, Homo erectus.

  • The overall shape of the teeth looked odd to me,

  • so I performed crown-shape analysis

  • on the occlusal surfaces of deciduous teeth, or baby teeth --

  • on your left --

  • and the permanent premolars and molars on your right.

  • The deciduous teeth are especially narrow,

  • and the premolars are unique in their outline shape

  • compared to other hominids.

  • In fact, when I compare the outlines,

  • when I lay them on top of each other,

  • they look very similar.

  • We say they have \"low intraspecific variations,\"

  • so the variation within the species is low.

  • When I compare this to groups like the australopithecines,

  • the intraspecific variation is much larger.

  • Postcranially, the team concluded

  • that the position of the shoulders suggesting naledi was a climber;

  • the flared pelvis and curved fingers are all primitive for the genus Homo.

  • On the other hand,

  • the humanlike wrist, long slender legs and modern feet

  • are all consistent with other members of the genus.

  • In 2017, we announced more specimens of Homo naledi

  • from the nearby Lesedi Chamber,

  • also in the Rising Star cave system.

  • In addition, our geology team managed to produce an age estimate.

  • The date's a big deal because, up until now,

  • we had based our analysis solely on the morphology of the specimens,

  • without previous knowledge of how old something is --

  • something which could unconsciously bias our interpretations.

  • With its small brain and flared pelvis,

  • we would not have been surprised

  • if the fossils turned out to be two million years old.

  • Instead, the fossils dated

  • to 235 to 336 thousand years,

  • an incredibly young date for such a small-brained individual.

  • So think back to what I said earlier:

  • we thought that our brains were becoming larger relative to the rest of our body.

  • Now we have a small-brained, young individual complicating this idea.

  • What does all this mean?

  • Homo naledi has taught us

  • that we need to reassess what it means to be in the genus Homo.

  • We need to rethink what it means to be human.

  • In fact, most of the characteristics that we use to define the genus Homo,

  • such as brain size and hip morphology,

  • are no longer valid.

  • No other species exists with this mix of primitive and derived traits.

  • Why is there so much morphological variation in the genus Homo?

  • And what force is driving that variation?

  • Another implication for these fossils is that for the first time,

  • we have concrete evidence of a species coexisting in Africa,

  • at 300,000 years,

  • with modern humans.

  • Until this discovery,

  • we only had large-brained modern humans that existed in Africa.

  • Did they interbreed with each other?

  • Did they compete with each other?

  • Another implication that these fossils have

  • is for the archaeologists studying stone tools in South Africa.

  • Keep in mind that neither the Dinaledi nor the Lesedi Chambers

  • have any artifacts in them.

  • However, they do overlap in time with several stone-tool industries,

  • the makers of which are considered to be either modern humans

  • or direct human ancestors.

  • This begs the question:

  • Who made the stone tools of South Africa?

  • Brain size has historically played a key role

  • in identifying a species as a tool user.

  • The idea is that you need to have a large brain

  • to have even the capacity to make stone tools.

  • But that notion has been questioned.

  • Furthermore, Homo naledi, even with its small brain size,

  • has a hand-wrist morphology similar to other species

  • that did make and use stone tools,

  • suggesting it had the capability.

  • With two species coexisting in Africa at 300,000 years,

  • we can no longer assume we know the maker of tools

  • at sites with no associated species.

  • So where does Homo naledi fit in our human evolutionary lineage?

  • Who is it most closely related to?

  • Who did it evolve from?

  • We're still trying to figure all that out.

  • It's ironic, because paleoanthropologists are renowned

  • for having small sample sizes.

  • We now have a large sample size,

  • and more questions than answers.

  • Homo naledi has taught us,

  • has brought us a little bit closer

  • to better understanding our evolutionary past.

  • So while Mrs. Ples will always hold a special place in my heart,

  • she now shares that space with several thousand others.

  • (Laughter)

  • Thank you.

  • (Applause)

Translator: Leslie Gauthier Reviewer: Krystian Aparta

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