by Jan Bartek
An interesting piece of
fossil of extinct human species suggests climate change contributed
to the evolution and unexpected anatomical changes that previously
were attributed to sex.
The skull of
Paranthropus robustus who
walked the earth at roughly the same time as our direct ancestor
Homo erectus was discovered in the fossil-rich Drimolen cave system
in South Africa.
Males of the extinct human species Paranthropus robustus were
thought to be substantially larger than females, much like the size
differences seen in modern-day primates such as gorillas, orangutans
However, the 2-million-year-old skull of Paranthropus robustus
shows some gaps in the current theory of evolution.
"This is the type of
phenomenon that can be hard to document in the fossil record,
especially with respect to early human evolution," said David
Strait, professor of biological anthropology in Arts & Sciences
at Washington University.
Researchers already knew
that the appearance of P. robustus in South Africa roughly
coincided with the disappearance of
Australopithecus, a somewhat
more primitive early human, and the emergence in the region of early
representatives of Homo, the genus to which modern people
This transition took
place very rapidly, perhaps within only a few tens of thousands of
hypothesis has been that climate change created stress in
populations of Australopithecus leading eventually to their
demise, but that environmental conditions were more favorable
for Homo and Paranthropus, who may have dispersed into the
region from elsewhere," Strait said.
"We now see that
environmental conditions were probably stressful for
Paranthropus as well, and that they needed to adapt to survive."
The new specimen
Drimolen, identified as DNH 155
cranium DNH 155 documents Microevolution in an Early Hominin Species),
is clearly a male but differs in important ways from other P.
robustus previously discovered at the nearby site of Swartkrans -
where most of the fossils of this species have been found.
Evolution within a species can be difficult to see in the fossil
record. Changes may be subtle, and the fossil record is notoriously
Usually, the fossil record reveals larger-scale patterns, such as
when species or groups of species either appear in the fossil record
or go extinct.
So this Drimolen
discovery provides a rarely seen window into early human evolution.
The new specimen is larger than a well-studied member of the species
previously discovered at Drimolen - an individual known as
DNH 7, and presumed to be female -
but is measurably smaller than presumed males from Swartkrans.
"It now looks as if
the difference between the two sites cannot simply be explained
as differences between males and females, but rather as
population-level differences between the sites," said Jesse
Martin, a doctoral student at La Trobe University and the
co-first author of the study.
"Our recent work has
shown that Drimolen predates Swartkrans by about 200,000 years,
so we believe that P. robustus evolved over time, with Drimolen
representing an early population and Swartkrans representing a
later, more anatomically derived population."
"One can use the fossil record to help reconstruct the
evolutionary relationships between species, and that pattern can
provide all sorts of insights into the processes that shaped the
evolution of particular groups," Martin said.
"But in the case of
P. robustus, we can see discrete samples of the species drawn
from the same geographic region but slightly different times
exhibiting subtle anatomical differences, and that is consistent
with change within a species."
"It's very important to be able to document evolutionary change
within a lineage," said Angeline Leece of La Trobe University,
the other first author of the study.
"It allows us to ask
very focused questions about evolutionary processes. For
example, we now know that tooth size changes over time in the
species, which begs the question of why.
There are reasons to
believe that environmental changes placed these populations
under dietary stress, and that points to future research that
will let us test this possibility."
Co-director of the
Drimolen project, La Trobe University's Andy Herries
"Like all other
creatures on earth, our ancestors adapted and evolved in
accordance with the landscape and environment around them.
For the first time in
South Africa, we have the dating resolution and morphological
evidence that allows us to see such changes in an ancient
hominin lineage through a short window of time."
of Climate Change
The evidence of rapid but significant climate change during this
period in South Africa comes from a variety of sources.
indicate that certain mammals associated with woodland or bushland
environments went extinct or became less prevalent - while other
species associated with drier, more open environments appeared
locally for the first time.
"P. robustus is
remarkable in that it possesses a number of features in its
cranium, jaws and teeth indicating that it was adapted to eat a
diet consisting of either very hard or very tough foods," Strait
"We think that these
adaptations allowed it to survive on foods that were
mechanically difficult to eat as the environment changed to be
cooler and drier, leading to changes in local vegetation.
"But the specimens from Drimolen exhibit skeletal features
suggesting that their chewing muscles were positioned in such a
way as to make them less able to bite and chew with as much
force as the later P. robustus population from Swartkrans," he
"Over the course of
200,000 years, a dry climate likely led to natural selection
favoring the evolution of a more efficient and powerful feeding
apparatus in the species."
A 2-million-year-old skull of a large-toothed,
small-brained distant human cousin in South Africa.
Credit: Australian Associated Press
Angeline Leece said it was notable that P. robustus appeared
at roughly the same time as our direct ancestor Homo erectus, as
documented by an infant H. erectus cranium that the team discovered
at the same Drimolen site in 2015.
"These two vastly
different species, H. erectus with their relatively large brains
and small teeth, and P. robustus with their relatively large
teeth and small brains, represent divergent evolutionary
experiments," Leece said.
"While we were the
lineage that won out in the end, the fossil record suggests that
P. robustus was much more common than H. erectus on the
landscape two million years ago."
More broadly, the
researchers think that this discovery serves as a cautionary tale
for recognizing species in the fossil record.
A large number of fossil human species have been discovered over the
past quarter century, and many of these new species designations are
based on a small number of fossils from only one or a few sites in
small geographic areas and narrow time ranges.
"We think that paleo-anthropology
needs to be a bit more critical about interpreting variation in
anatomy as evidence for the presence of multiple species,"
"Depending on the
ages of fossil samples, differences in bony anatomy might
represent changes within lineages rather than evidence of