https://phys.org/news/2024-03-migration-hominins-africa-driven-major.html

A pair of planetary scientists, one with the
University of Milan, the other with Columbia
University, has found evidence that the exodus
of hominins out of Africa approximately 1
million years ago may have been driven by the
first major glaciation of the Pleistocene.

In their study, reported in Proceedings of the
National Academy of Sciences, Giovanni Muttonia
and Dennis Kent more accurately dated the onset
of the first major Pleistocene ice age and
compared it with genetic evidence of a hominin
population bottleneck described in prior
research efforts.

Prior research has shown that a major migration
of hominins out of Africa occurred sometime
between 1.1 and 0.9 million years ago. Research
has also suggested that there was a hominin
population bottleneck (drop in numbers) roughly
around the same time that triggered the migration.
In this new study, the researchers sought to
better explain the timing and reason for the
migration.

The team began by studying shifts in oxygen
isotopes (found in rock sediment layers), which
allowed them to see that the first major
Pleistocene began approximately 900,000 years
ago. They turned their attention to the results
of prior studies that showed a population
bottleneck approximately 200,000 years earlier.
In that work, the team found that the results
were not reliable—it is possible, they note,
that population numbers were higher but there
were areas where they were not being counted.

They then pointed out that evidence in past
research showed hominin habitation all across
Eurasia started approximately 900,000 years
ago, which coincides with the onset of the
first Pleistocene ice age. As the ice age
began, ocean levels would have dropped,
allowing hominins an easier route from
Africa. Also, conditions in Africa would
have become more difficult for the hominins
living there, making migration a tempting
proposition. And the researchers note that
many animals also began migrating out of
Africa around the same time.


https://www.pnas.org/doi/10.1073/pnas.2318903121
Hominin population bottleneck coincided with
migration from Africa during the Early
Pleistocene ice age transition

Significance
The timing and causes of hominin (pre-Homo
sapiens) migrations out of Africa have been
of recent interest. Two scenarios, one based
on modern genomic data and the other on the
chronology of hominin sites, indicate
population bottlenecking in the Early
Pleistocene. An ice age is invoked as
bottleneck trigger in both cases even though
they differ in timing, and therefore in the
actual event that triggered depopulation.
Our assessment of the chronology of key
hominin sites in Eurasia leads us to
conclude that bottlenecking occurred at the
first major ice age of the Pleistocene,
~900,000 y ago, in agreement with the genomic
model, and coincided with a major diaspora
from Africa into Eurasia when hominins came
close to extinction.

Abstract
Two recently published analyses make cases
for severe bottlenecking of human populations
occurring in the late Early Pleistocene, one
case at about 0.9 Mya based on a genomic
analysis of modern human populations and the
low number of hominin sites of this age in
Africa and the other at about 1.1 Mya based
on an age inventory of sites of hominin
presence in Eurasia. Both models point to
climate change as the bottleneck trigger,
albeit manifested at very different times,
and have implications for human migrations
as a mechanism to elude extinction at
bottlenecking. Here, we assess the climatic
and chronologic components of these models
and suggest that the several
hundred-thousand-year difference is largely
an artifact of biases in the
chronostratigraphic record of Eurasian
hominin sites. We suggest that the best
available data are consistent with the
Galerian hypothesis expanded from Europe to
Eurasia as a major migration pulse of fauna
including hominins in the late Early
Pleistocene as a consequence of the opening
of land routes from Africa facilitated by a
large sea level drop associated with the
first major ice age of the Pleistocene and
concurrent with widespread aridity across
Africa that occurred during marine isotope
stage 22 at ~0.9 Mya. This timing agrees
with the independently dated bottleneck
from genomic analysis of modern human
populations and allows speculations about
the relative roles of climate forcing on
the survival of hominins.




The team suggests that the true reason for the migration was climate change—and it happened approximately 0.9 million years ago.

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