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