Case study: 54
Syllabus:1.6 Phylogenetic status, characteristics and geographical
distribution of the following.
Source: National Geographic
Fossil skulls rewrite the stories of two ancient human ancestors Found in a hilltop cave, the oldest known Homo erectus and Paranthropus robustus fossils shed light on a critical period of hominin
evolution. In the winter of 2015, Jesse Martin and Angeline Leece were extracting what they thought were baboon remains from a piece of rock. The two students at La Trobe University in Australia were part of an expedition to collect and study fossils from the Drimolen quarry northwest of Johannesburg, South Africa. As they cleaned the skull fragments and pieced them back together, however, they realized the fossils did not come from a baboon, but instead comprised the braincase
of a young Homo erectus, a species never before identified in South Africa.
“I don’t think our supervisors believed us until they came over to have a look,” Martin recalls.
The braincase was described in the journal Science today, together with the skullcap of another ancient hominin, Paranthropus robustus, found at the same site. A suite of different dating techniques all hinted that the two species’ braincases were more or less the same age—
about two million years old. This would make them the earliest fossils ever found for their respective species, according to the new study coauthored by Martin and Leece. “I think they have made a strong case for the oldest Homo erectus in Africa, and in fact, in the world,” Lee Berger, a paleoanthropologist at the University of Witwatersrand said.
The enigmatic origins of Homo erectus The age of the fossils was particularly surprising for the Homo erectus skull. Most paleoanthropologists believe that this human ancestor arose in East Africa, where several younger Homo erectus fossils—as well as the likely remains of older Homo species—have been found. Some have even hypothesized that Homo erectus originated outside
of Africa, because the oldest known fossils from the species—before this new find—were discovered at the site of Dmanisi in Georgia.
An Asian origin for H. erectus now seems exceedingly unlikely, Martin says. “The first problem
for that idea is that the earliest evidence for Homo erectus is now from South Africa. But the
bigger problem is that there is no candidate ancestor for Homo erectus in Asia. If you dig any
deeper at sites where Homo erectus remains have been found, there are no hominins there.”
The discovery of the new braincase in South Africa, however, does not necessarily mean
that Homo erectus originated there either. “Based on the current evidence, my guess is it
emerged somewhere in Africa we haven’t looked yet,” Martin says.
Marcia Ponce de Léon, a paleoanthropologist at the University of Zürich in Switzerland who
was not involved in the new study, agrees that “it is reasonable to call the new fossil Homo
erectus.” A 2013 study by Ponce de Léon and colleagues described a 1.8-million-year-old hominin
skull from Dmanisi, Georgia, identifying it as likely belonging to one of the
earliest Homo erectus to leave Africa.
As the species migrated across continents, it continued to adapt to new environments. “Every
population of every species continues to evolve wherever it goes,” Ponce de Léon says. Tracing
the spread and adaptation of Homo erectus halfway across the globe could help scientists learn
more about the way our wandering ancestor survived in the different environments it encountered.

“This was really the first human experiment with globalization,” Martin says.Three hominins in southern Africa Two million years ago, Homo erectus wasn’t exactly abundant. “They appear to have been outnumbered 10 to 1 by Paranthropus robustus,” Leece says.
As its name indicates, Paranthropus robustus—one of the “robust australopithecines”—had a
very stout skull, particularly large teeth, and an impressive crest on top of the braincase where

its massive chewing muscles were attached. “The leading theory is that they were eating tough
foods—not necessarily things that needed crunching, but foods that were fibrous and require a
lot of chewing, like certain tubers or grasses,” Leece says.
The even older species Australopithecus sediba was also still roaming the region. The fossil
record suggests this is about when Australopithecus started to be replaced
by Paranthropus and Homo, a critical time in the evolution of our predecessor species.
For most of the year, Homo, Paranthropus, and Australopithecus species had plenty of available
resources, and all three were likely eating more or less the same things. But winters can be
harsh in this area, Martin says. “In the morning, it’s freezing, and according to estimates, it
would’ve been even colder then. So this was quite a tough climate for a hominin.” Under those
challenging circumstances, Paranthropus robustus’s powerful jaws and ability to eat tough, fibrous
foods probably provided it with a significant advantage.
One theory holds that Australopithecus sediba may have been a direct ancestor to the
genus Homo, including the species Homo erectus. The authors of the new study question this
theory, however, as the newfound Homo erectus skull is older than Australopithecus sediba remains
found at the nearby site of Malapa.
Berger, who was part of the team that found the Australopithecus sediba fossils at Malapa in
2010, believes that even though the Homo erectus skull is older, Australopithecus sediba still
could have been an ancestor to the species. “Mother species can easily exist at the same time
and place as their descendant species do,” he says.
Regardless of which of these species emerged first, one thing is clear: Over a million years later,
only Homo erectus still walked the Earth.
Homo erectus conquers the world While the hyperspecialized skull of Paranthropus robustus may have served it well in certain environments, the trait may have ultimately become its downfall, Leece says. When the environment changes, extreme adaptations can bec ome a handicap.
Comparing the two newly analyzed braincases, it becomes clear that Homo erectus, while initially
outcompeted by Paranthropus robustus, was working on a revolutionary adaptation of its
own. H. erectus’s characteristic tear-shaped braincase suggests the early member of
the Homo genus was expanding and reorganizing its brain.
The Homo erectus skull Martin and Leece wrested out of the rock did not belong to an adult.
Judging by the extent to which the bones of the skull had already fused, the braincase came
from a child between two and six years old. At this tender age, its brain would already have
been larger than that of most Australopithecus and Paranthropus adults. And impressions on the
fossils show that the child’s brain was still growing, pushing the skull bones outward. “We can
even see blood vessels,” Martin says.
Whereas Paranthropus robustus evolved a kind of “portable grinding stone,” Homo
erectus “adapted to be adaptable” and to solve all kinds of problems that it would have encountered along its journey from Africa to Asia and parts of southern Europe, Martin says. The
species’ increasingly nimble brain allowed it to outsmart other animals by fashioning tools, collaborating with others, and perhaps even pondering the future.
Homo erectus survived for nearly two million years, making it the most successful species
of Homo ever known.