The age of the oldest fossils in eastern Africa widely recognized as
representing our species, Homo sapiens, has long been uncertain. Now, dating
of a massive volcanic eruption in Ethiopia reveals they are much older than
previously thought.
The remains—known as Omo I—were found in Ethiopia in the late 1960s, and
scientists have been attempting to date them precisely ever since, by using
the chemical fingerprints of volcanic ash layers found above and below the
sediments in which the fossils were found.
An international team of scientists, led by the University of Cambridge, has
reassessed the age of the Omo I remains—and Homo sapiens as a species.
Earlier attempts to date the fossils suggested they were less than 200,000
years old, but the new research shows they must be older than a colossal
volcanic eruption that took place 230,000 years ago. The results are
reported in the journal Nature.
The Omo I remains were found in the Omo Kibish Formation in southwestern
Ethiopia, within the East African Rift valley. The region is an area of high
volcanic activity, and a rich source of early human remains and artifacts
such as stone tools. By dating the layers of volcanic ash above and below
where archaeological and fossil materials are found, scientists identified
Omo I as the earliest evidence of our species, Homo sapiens.
"Using these methods, the generally accepted age of the Omo fossils is under
200,000 years, but there's been a lot of uncertainty around this date," said
Dr. Céline Vidal from Cambridge's Department of Geography, the paper's lead
author. "The fossils were found in a sequence, below a thick layer of
volcanic ash that nobody had managed to date with radiometric techniques
because the ash is too fine-grained."
As part of a four-year project led by Professor Clive Oppenheimer, Vidal and
her colleagues have been attempting to date all the major volcanic eruptions
in the Ethiopian Rift around the time of the emergence of Homo sapiens, a
period known as the late Middle Pleistocene.
The researchers collected pumice rock samples from the volcanic deposits and
ground them down to sub-millimeter size. "Each eruption has its own
fingerprint—its own evolutionary story below the surface, which is
determined by the pathway the magma followed," said Vidal. "Once you've
crushed the rock, you free the minerals within, and then you can date them,
and identify the chemical signature of the volcanic glass that holds the
minerals together."
The researchers carried out new geochemical analysis to link the fingerprint
of the thick volcanic ash layer from the Kamoya Hominin Site (KHS ash) with
an eruption of Shala volcano, more than 400 kilometers away. The team then
dated pumice samples from the volcano to 230,000 years ago. Since the Omo I
fossils were found deeper than this particular ash layer, they must be more
than 230,000 years old.
"First I found there was a geochemical match, but we didn't have the age of
the Shala eruption," said Vidal. "I immediately sent the samples of Shala
volcano to our colleagues in Glasgow so they could measure the age of the
rocks. When I received the results and found out that the oldest Homo
sapiens from the region was older than previously assumed, I was really
excited."
"The Omo Kibish Formation is an extensive sedimentary deposit which has been
barely accessed and investigated in the past," said co-author and co-leader
of the field investigation Professor Asfawossen Asrat from Addis Ababa
University in Ethiopia, who is currently at BIUST in Botswana. "Our closer
look into the stratigraphy of the Omo Kibish Formation, particularly the ash
layers, allowed us to push the age of the oldest Homo sapiens in the region
to at least 230,000 years."
"Unlike other Middle Pleistocene fossils which are thought to belong to the
early stages of the Homo sapiens lineage, Omo I possesses unequivocal modern
human characteristics, such as a tall and globular cranial vault and a
chin," said co-author Dr. Aurélien Mounier from the Musée de l'Homme in
Paris. "The new date estimate, de facto, makes it the oldest unchallenged
Homo sapiens in Africa."
The researchers say that while this study shows a new minimum age for Homo
sapiens in eastern Africa, it's possible that new finds and new studies may
extend the age of our species even further back in time.
"We can only date humanity based on the fossils that we have, so it's
impossible to say that this is the definitive age of our species," said
Vidal. "The study of human evolution is always in motion: boundaries and
timelines change as our understanding improves. But these fossils show just
how resilient humans are: that we survived, thrived and migrated in an area
that was so prone to natural disasters."
"It's probably no coincidence that our earliest ancestors lived in such a
geologically active rift valley—it collected rainfall in lakes, providing
fresh water and attracting animals, and served as a natural migration
corridor stretching thousands of kilometers," said Oppenheimer. "The
volcanoes provided fantastic materials to make stone tools and from time to
time we had to develop our cognitive skills when large eruptions transformed
the landscape."
"Our forensic approach provides a new minimum age for Homo sapiens in
eastern Africa, but the challenge still remains to provide a cap, a maximum
age, for their emergence, which is widely believed to have taken place in
this region," said co-author Professor Christine Lane, head of the Cambridge
Tephra Laboratory where much of the work was carried out. "It's possible
that new finds and new studies may extend the age of our species even
further back in time."
"There are many other ash layers we are trying to correlate with eruptions
of the Ethiopian Rift and ash deposits from other sedimentary formations,"
said Vidal. "In time, we hope to better constrain the age of other fossils
in the region."
Reference:
Céline Vidal, Age of the oldest known Homo sapiens from eastern Africa,
Nature (2022).
DOI: 10.1038/s41586-021-04275-8.