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For about a million years in the early Pleistocene
Epoch -- from about 2.4 million to 1.4 million
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years ago -- it was a really exciting time
to be a hominin.
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Hominins, you’ll recall, are the group of
human ancestors that are more closely related
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to us than to chimps and bonobos.
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During this million-year stretch, different
branches of our evolutionary tree were flourishing
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all over southern and eastern Africa.
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And if we were to zoom in on the earliest
part of this million years, we’d encounter
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a familiar face.
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Or at least a somewhat familiar face: the
face of Homo habilis.
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It stood just over a meter tall, and had a
slightly larger brain and smaller teeth than
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its earlier relatives, the australopithecines.
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But it still had longer arms and a protruding
lower face, traits that are usually considered
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more basal in the hominin lineage.
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And yet!
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This ancestor probably made and used stone
tools!
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In fact, Homo habilis means “handy man,”
and its discoverers gave it that name because
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they thought that it was responsible for the
many tools that had been found near its remains.
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But, does this hominin really belong in our
genus, the genus Homo?
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Was it more like us than its earlier ancestors?
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Over the last fifty years or so, the human
family tree has really filled out.
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We’ve discovered all kinds of new fossils
of our ancestors and relatives, like australopithecines
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that have about the same brain size and limb
proportions as Homo habilis.
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And this has led some researchers to question
whether Homo habilis is really a member of
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our genus at all.
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As more and more fossil ancestors have been
found, our genus has become more and more
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inclusive, incorporating more members that
look less like us, Homo sapiens.
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And this is an important problem to think
about.
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Because, there's some consensus about who
belongs in our immediate human family -- like
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us, neanderthals, and even the ancient, globe-traveling
hominin Homo erectus.
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They’re all agreed to be clear-cut members
of the genus Homo.
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But beyond them, there are lots ancestors
for whom we can’t find a home.
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And there is no official definition of what
constitutes a human, either, whether that
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means being a member of our genus, or our
species, or just being able to walk upright
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and make tools.
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So by getting to know these other hominins
-- the ones who came before us, the neanderthals,
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and our other contemporaries -- we can start
to answer some big, interesting, and difficult
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questions …
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….questions about what it essentially means
to be human.
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When the first fossils of Homo habilis were
found by Louis and Mary Leakey’s team in
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the 1960s, they had a difficult choice to
make:
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Were these the remains of australopithecines?
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Or were they in fact the earliest known members
of our own genus, Homo?
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Traditionally, defining who belongs in our
genus has come down to which traits are considered
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“uniquely” human.
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And when the Leakeys were pondering Homo habilis,
they used a definition of Homo from 1955,
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which said that to be a member of the genus,
you had to have some number of features in
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common with the three members of Homo known
at the time: Homo sapiens, Homo erectus, and
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the Neanderthals.
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The Leakeys decided that Homo habilis shared
three important traits with the other members
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of our genus: It had an upright posture; it
was bipedal, and it had the manual dexterity
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to make tools.
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And, sure, Homo habilis had those three things.
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But in the decade after Homo habilis was found,
new discoveries of other human ancestors were
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made in the same parts of Africa, and they
had these traits, too.
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And these new finds were all of various australopithecines,
which were inarguably not part of our genus.
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The most of famous of these discoveries is
the specimen known as Lucy.
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Unearthed at Hadar, Ethiopia in 1974, she
was one of the most complete specimens of
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Australopithecus afarensis ever found.
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And she gave clear evidence of an upright
posture, like having thigh bones that angled
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inward toward the knee and a more human-like
pelvis.
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Then, four years later, a set of fossilized
footprints were found in Tanzania.
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Known as the Laetoli footprints, they were
probably made by Australopithecus afarensis,
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too -- again showing that hominins were walking
on two feet more than a million years before
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Homo habilis was around.
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So if walking upright was not exclusive to
our genus, then the definition of our genus
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had to change.
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Instead of just physical traits, the thinking
then turned toward lifestyle adaptations as
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a way of defining who belonged in our group.
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Lifestyle adaptations are features that are
linked to how a hominin lived its life, like
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what it ate, how it got around, and where
it lived.
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For example, the increased brain size in members
of Homo was thought to be linked to a higher-quality
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diet, because being able to consume more calories
more efficiently has allowed for larger brains.
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And some researchers arrived at four specific
lifestyle adaptations that they thought might
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qualify a hominin for entry into the genus
Homo.
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Those adaptations included: an adult brain
size greater than 600 cubic centimeters; limb
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proportions similar to ours, with long legs
compared to our arms; the use of language;
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and the manufacture and use of stone tools.
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But still, these things only kind of applied
to Homo habilis.
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Because one of the most famous and complete
Homo habilis skulls, a specimen known as KNM-ER
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1813, had a cranial capacity of only 510 ccs.
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Meanwhile, a big male specimen of Australopithecus
afarensis was found to have had limb proportions
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like those of early members of Homo -- but
it lived 3.58 million years ago, way before
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Homo habilis appeared on the scene.
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And the capability for language can really
only be inferred from the fossil record.
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It’s pretty hard to tell whether Homo habilis
or any ancestor that lived millions of years
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ago was able to speak.
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That just leaves stone tools.
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And while researchers in the 1960s were pretty
convinced that Homo habilis was the maker
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of the tools at Olduvai Gorge, we now know
that australopithecines could likely make
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stone tools, too.
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So, let’s look at our group another way.
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Instead of talking about who might not belong
to our genus, let’s consider who might.
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Who were those other members of our genus
that lived alongside Homo habilis during that
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exciting, million-year span in Africa?
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And what can they tell us about the origin
of the Homo genus?
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Well, starting about 1.98 million years ago
in South Africa, there lived an australopithecine
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with distinctly Homo-like traits.
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Known from several relatively complete skeletons,
it was given the name in 2010 of Australopithecus
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sediba.
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Its discoverers placed it in Australopithecus
because of its small brain and long arms,
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but they also noted that it had small molars
and premolars, and facial features that were
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similar to other Homo specimens.
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So these researchers actually think that Australopithecus
sediba might be more closely related to our
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genus than other australopithecines are, but
other experts think it’s too recent in age.
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Another candidate for inclusion?
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Homo rudolfensis
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It’s been found at sites dating back 1.8
to 1.9 million years ago in Eastern Africa.
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The best fossil of this species is known as
KNM-ER-1470, and when it was discovered in
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1972, it was originally classified as a large
specimen of Homo habilis.
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However in 1986 and again in 1992, further
studies found that its bigger brain, longer
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face, and larger premolars and canines made
it too different from Homo habilis to be a
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member of that species.
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But it was still assigned to our genus, because
of its big brain.
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At 775 cubic centimeters, it was well over
the classic 600cc cut-off.
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And finally we come to the first indisputable
member of our genus, and one of the most successful
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and widespread: Homo erectus.
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It lived from 1.9 million to just 143,000
years ago!
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The first Homo erectus fossils were found
in 1891, and some anthropologists later split
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this species into two - with Homo erectus
including the later African and Asian fossils,
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and the earlier African fossils being filed
under Homo ergaster.
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And experts generally agree that Homo erectus
is definitely a member of our genus.
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These hominins had modern human-like proportions,
were potentially capable of long-distance
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running, and generally had much smaller molars
and much larger brains than their predecessors.
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In other words, they were a lot more like
us than any of the other species I’ve mentioned
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so far.
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Homo erectus is also the first species that
we have fossil evidence for outside of Africa.
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They made it as far as China and Indonesia,
but their initial foray seems to have landed
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them in the Republic of Georgia, at a site
called Dmanisi that dates to about 1.77 million
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years ago.
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And the interesting thing about that site
is that there’s a lot of variation among
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the specimens found there.
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Some individuals from Dmanisi had the unmistakable
brow ridge of Homo erectus, but their brains
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were smaller than 600 ccs -- the classic cut-off
for inclusion in the genus Homo.
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In fact, there’s so much variation in the
Georgian fossils that their discoverers made
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a case in 2013 for taking all of the other
early Homo fossils -- including the ones assigned
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to Homo habilis and Homo rudolfensis -- and
putting them in Homo erectus, lumping everything
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together as a single species.
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They argue that if the fossils from a single
site can show as much variation as we find
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between species, then all of those early groups
might as well be considered the same species.
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But of course, other experts disagree.
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They don’t think overall cranial shape is
enough to distinguish one species from another.
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To them, the devil is in the differing morphological
details of each skull.
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Now, with all this in mind, let’s go back
to Homo habilis.
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Where does it belong?
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Well, it doesn’t really seem to fit anyone’s
definition of our genus.
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And the best argument for keeping it in is
just that taking it out would require redefining
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what it means to be a member, which would
be a major taxonomic undertaking.
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Some experts have proposed lumping habilis
into the genus Australopithecus.
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Others say it’s neither Homo nor Australopithecus
and that it deserves its own new genus.
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So far, no single opinion has won out.
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Homo habilis remains a taxon in limbo.
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Ultimately, what defines our genus comes down
to how much variation in morphology, time,
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and space we’re willing to include in the
group we call home.
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In the past, an increase in brain size, a
bipedal gait, human-like limb proportions,
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and tool use seemed to have been enough for
inclusion.
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Those are the things that we thought made
us members of the same genus.
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But as we’ve discovered more and more hominin
fossils, our family tree has become more complicated,
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rather than less.
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So now, the latest research is suggesting
totally new ways to define our lineage.
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One new idea for a defining feature of our
genus?
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Tooth size!
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Smaller teeth generally indicate a higher
quality diet and the ability to prepare food
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with tools, instead of having to chew tough
foods for a long time.
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Another possible criterion is the pace of
our development.
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We modern humans have longer periods of childhood
and adolescence compared to our closest ape
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relatives, because we need that time to grow
our large brains and use them to learn.
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And we can track these growth patterns in
fossils by studying microscopic features of
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teeth.
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And as recently as 2015, some experts have
suggested that we should scrap the whole list
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of hominins altogether and just start from
scratch.
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They say we should step back and look at the
totality of the fossil record with fresh eyes
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to decide what traits we think are important
for being “human.”
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As it stands, there’s still no single way
to define our genus.
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Mostly it happens by comparison: Is a new
fossil more like what we’ve called Homo
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in the past or is it more like an australopithecine?
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And the jury’s still out on Homo habilis,
the species that started all the trouble in
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the first place.
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But if anything, the trouble really began
back in the Early Pleistocene, during that
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exciting million years or so when this group
of hominins first started to flourish.
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And it may be in the fossils from that time
-- perhaps in fossils we haven’t found yet
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-- that will help us better answer the question
of who belongs to our very exclusive group.
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Thanks as always, and extra big thanks to
our current Eontologists, Jake Hart, Jon Ivy,
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John Davison Ng and STEVE!
00:12:02
If you’d like to join them and our other
patrons in supporting what we do here, then
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go to patreon.com/eons and make your pledge!
00:12:10
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