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