How a new species of ancestors is changing our theory of human evolution | Juliet Brophy

301,755 views ・ 2019-03-19

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Translator: Leslie Gauthier Reviewer: Krystian Aparta
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Human origins.
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Who are we?
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Where do we come from,
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and how do we know?
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In my field, paleoanthropology,
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we explore human origins --
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the "who" and "where" questions --
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by analyzing fossils that date back thousands and even millions of years.
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In 2015, a team of colleagues and I named a new species in the genus Homo --
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our genus --
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Homo naledi.
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Let's take a step back and put that into context.
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The last common ancestors between humans and chimps
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date somewhere between six and eight million years.
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The earliest hominins,
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or earliest human ancestors,
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evolved into a group known as the australopithecines.
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The australopithecines evolved into the genus Homo
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and eventually modern humans -- us.
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With each new fossil discovery,
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we get a little bit closer to better understanding who we are
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and where we came from.
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With these new fossil finds,
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we realize we now have to make changes to this tree.
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Until this discovery,
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we thought we had a pretty good idea about the patterns of evolutionary change.
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Current fossil evidence suggests
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that the earliest populations of the genus Homo evolved in Africa
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somewhere between two and three million years.
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Fast-forward to approximately 300,000 years to where we see the origins
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of the first modern humans.
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While the fossil record between these time frames in Africa
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is relatively sparse,
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the fossils nonetheless demonstrated certain trends
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from our earliest ancestors to modern humans.
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For example, our brains were becoming larger
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relative the rest of our body.
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Our pelves were becoming more bowl-shaped,
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and our hand-wrist morphology, or form,
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suggested a change in our grip as we began to make and use stone tools
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and spend less time in the trees.
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These new fossils disrupt everything we thought we knew about these trends
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and force us to change the way that we think about human evolution.
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South Africa in general,
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but the Cradle of Humankind in particular,
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contains numerous sites where hundreds of thousands of fossils have been found.
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As an undergraduate student, I fell in love with one of them ...
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Mrs. Ples.
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The skull of a 2.1-million-year-old early human ancestor.
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From that point on,
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I was determined to go to South Africa and study human evolution.
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I first traveled there in 2003,
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and I did get to see my beloved Mrs. Ples.
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(Laughter)
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But words can hardly convey my excitement
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when I was chosen as an early career scientist
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by Lee Berger,
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a world-renowned paleoanthropologist,
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to be one of the primary analysts of recently excavated unpublished fossils.
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This treasure trove of fossils was being recovered from a new site
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called the Dinaledi Chamber in the Rising Star cave system.
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Species are often named based on a skull, a lower jaw,
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or, very rarely,
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a handful of postcranial, or below-the-neck, elements.
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The fossils from Dinaledi were another story altogether.
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An unprecedented approximately 1800 specimens --
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so far --
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have been excavated from the Rising Star system,
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representing at least 15 individual skeletons.
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The research team that I was invited to join
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was tasked with describing, comparing and analyzing the fossils,
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with the difficult goal of identifying to what species the fossils belonged.
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We were divided up into our different areas of expertise.
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We were divided up in different areas of the lab, too.
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So there was "Hand Land," for the fossil hand people,
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"Hip Heaven" for the pelvis ...
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I was in the "Tooth Booth."
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(Laughter)
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And after long, intense days in the lab,
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the different teams would meet up at night and discuss our findings,
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still consumed by questions from our analyses.
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It was incredible how different the interpretations were.
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Each body part seemed to come from a different species,
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based on what we knew from the fossil record.
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The suite of characteristics we were seeing didn't match any known species.
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And if we had only recovered the skull, we might have called it one thing;
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if we had only recovered the pelvis, we might have called it another.
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The anatomy of the skeletons didn't make sense
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with the framework of what we thought we knew of human evolution.
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Did it belong in the genus Homo?
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Should it be an australopithecine?
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Those bipedal, more apelike ancestors?
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Or perhaps it should be its own species.
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Ultimately, after much deliberation,
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we decided the Rising Star specimens did indeed warrant a new species,
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which we called "Homo naledi."
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From the head to the feet,
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the fossils present a mosaic of primitive, or ancestral,
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and derived or more modern-like features.
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The skull is quite derived,
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appearing most similar to early representatives of the genus Homo,
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like Homo habilis and Homo erectus.
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However, the brain is scarcely half the size of a modern human one.
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One that is smaller than any other early Homo that has ever been found.
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As someone who studies teeth,
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I might argue these are the coolest fossils found at the site.
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(Laughter)
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The assemblage consists of 190 whole or fragmentary teeth
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that range in age from very old to very young.
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Like the skeletons,
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the teeth present a mix of primitive and derived traits.
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In modern humans,
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the third molar is typically the smallest, while the first molar is the biggest,
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but Homo naledi has the primitive condition
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where the third molar is the biggest and the first molar is the smallest.
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The anterior teeth,
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or the incisors and canines,
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are small for the genus Homo,
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and the lower canine has a cuspulid on it --
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an extra cuspule that gives it a distinct mitten-like shape
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that it shares with some specimens of the early human, Homo erectus.
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The overall shape of the teeth looked odd to me,
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so I performed crown-shape analysis
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on the occlusal surfaces of deciduous teeth, or baby teeth --
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on your left --
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and the permanent premolars and molars on your right.
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The deciduous teeth are especially narrow,
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and the premolars are unique in their outline shape
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compared to other hominids.
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In fact, when I compare the outlines,
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when I lay them on top of each other,
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they look very similar.
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We say they have "low intraspecific variations,"
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so the variation within the species is low.
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When I compare this to groups like the australopithecines,
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the intraspecific variation is much larger.
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Postcranially, the team concluded
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that the position of the shoulders suggesting naledi was a climber;
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the flared pelvis and curved fingers are all primitive for the genus Homo.
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On the other hand,
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the humanlike wrist, long slender legs and modern feet
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are all consistent with other members of the genus.
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In 2017, we announced more specimens of Homo naledi
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from the nearby Lesedi Chamber,
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also in the Rising Star cave system.
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In addition, our geology team managed to produce an age estimate.
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The date's a big deal because, up until now,
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we had based our analysis solely on the morphology of the specimens,
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without previous knowledge of how old something is --
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something which could unconsciously bias our interpretations.
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With its small brain and flared pelvis,
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we would not have been surprised
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if the fossils turned out to be two million years old.
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Instead, the fossils dated
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to 235 to 336 thousand years,
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an incredibly young date for such a small-brained individual.
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So think back to what I said earlier:
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we thought that our brains were becoming larger relative to the rest of our body.
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Now we have a small-brained, young individual complicating this idea.
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What does all this mean?
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Homo naledi has taught us
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that we need to reassess what it means to be in the genus Homo.
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We need to rethink what it means to be human.
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In fact, most of the characteristics that we use to define the genus Homo,
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such as brain size and hip morphology,
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are no longer valid.
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No other species exists with this mix of primitive and derived traits.
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Why is there so much morphological variation in the genus Homo?
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And what force is driving that variation?
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Another implication for these fossils is that for the first time,
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we have concrete evidence of a species coexisting in Africa,
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at 300,000 years,
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with modern humans.
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Until this discovery,
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we only had large-brained modern humans that existed in Africa.
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Did they interbreed with each other?
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Did they compete with each other?
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Another implication that these fossils have
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is for the archaeologists studying stone tools in South Africa.
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Keep in mind that neither the Dinaledi nor the Lesedi Chambers
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have any artifacts in them.
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However, they do overlap in time with several stone-tool industries,
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the makers of which are considered to be either modern humans
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or direct human ancestors.
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This begs the question:
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Who made the stone tools of South Africa?
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Brain size has historically played a key role
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in identifying a species as a tool user.
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The idea is that you need to have a large brain
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to have even the capacity to make stone tools.
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But that notion has been questioned.
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Furthermore, Homo naledi, even with its small brain size,
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has a hand-wrist morphology similar to other species
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that did make and use stone tools,
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suggesting it had the capability.
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With two species coexisting in Africa at 300,000 years,
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we can no longer assume we know the maker of tools
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at sites with no associated species.
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So where does Homo naledi fit in our human evolutionary lineage?
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Who is it most closely related to?
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Who did it evolve from?
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We're still trying to figure all that out.
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It's ironic, because paleoanthropologists are renowned
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for having small sample sizes.
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We now have a large sample size,
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and more questions than answers.
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Homo naledi has taught us,
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has brought us a little bit closer
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to better understanding our evolutionary past.
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So while Mrs. Ples will always hold a special place in my heart,
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she now shares that space with several thousand others.
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(Laughter)
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Thank you.
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(Applause)
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