Author : Wahid Ahmad
Scientists believe we
separated from our closest relatives, chimpanzees and bonobos, over 6 million
years ago. But the species we call homo sapiens is thought to have first
appeared in Africa between 300,000 and 200,000 years ago. Since then, humans
have spread all over the world. About 2.8 million years ago, the first
members of our genus Homo appeared in East Africa. Homohabilis, one of the
earliest species, lived from roughly 2.4 million years ago to 1.4 million years
ago, and made simple old stone tools, small flakes, and choppers for cutting
and pounding animal carcasses and plant materials. By about 2 million years
ago, a descendant lineage gave rise to Homo erectus in Africa. The oldest
securely dated Homo erectus cranium from Dreolan, South Africa is about 2
million years ago. And by 1.8 million years ago, these populations had
dispersed into Eurasia. For example, Demoni in Georgia at 1.85 million years
ago. Homo erectus showed a leap in brain size, fully modern body proportions,
and a more sophisticated Aulian tool set, including large bacial handaxes. Over
the next million years, Homo erectus diversified regionally. African forms
sometimes called Homoaster persisted alongside Asian populations like Java man
and peing man well into the middle pleaene. In Africa and Europe, populations
gradually evolved larger brain cases and more rounded skulls, setting the stage
for a new form by about 600,000 years ago. That new form Homohylebergensis is
recognized in fossils dating between 600,000 and 200,000 years ago. Most
famously the Mau jaw in Germany and the Cobwway cranium in Zambia 500 to
300,000 years ago. Homohidlebergensis populations built simple shelters, used
wooden and stone tipped spears for hunting, and show evidence of fire use after
about 400,000 years ago. By about 400,000 years ago, European homohy
highlebergenses gave rise to the Neanderthalss, while African homohylebergis
lineages evolved into anatomically modern homo sapiens around 300,000 years
ago. The earliest widely accepted Homo sapiens fossils, those from Jebel
Earhode, Morocco, are dated to 300,000 years ago, pushing back the origin of
our species across much of Africa. Subsequent dispersals and episodes of
interbreeding with Neanderthalss and Dennisovvens would further shape the genetic
tapestry of all later humans. Using DNA from people living today, scientists
have learned a lot about recent human history. How our ancestors spread across
different continents, for example. But we still don't know much about what
happened long before modern humans appeared during the earlier phases of the
human lineage especially in the pletosene period which lasted from about 2.5
million to 12,000 years ago. This early history is important if we want to
fully understand where we came from. One of the biggest challenges is that
ancient DNA from Africa older than homo sapiens is extremely rare or missing
because it doesn't preserve well in hot climates. So instead of ancient DNA,
scientists try to look for clues in the DNA of people living today. The DNA of
modern humans still carries signals of population changes from the distant
past. One of the ways scientists study this is through something called the
site frequency spectrum, a tool that looks at how common or rare different
mutations are in the DNA of a population. These patterns can tell us how big or
small populations were in the past and when they may have gone through big
changes like sudden shrinkage or growth. Recently, scientists used advanced
mathematical modeling on DNA from today's populations to figure out what likely
happened in the past. They uncovered a shocking chapter in our history, a time
when the human population dropped to dangerously low numbers. This research
shows that our ancestors went through a major population bottleneck in the
past, reducing the number of breeding individuals to just around a thousand.
That's an incredibly small number. So small it nearly led to extinction. They
studied change in population size over time by analyzing patterns in modern
DNA. The method was applied to DNA from 10 African and 40 non-African
populations, focusing on parts of the genome that aren't influenced much by
natural selection or sequencing errors. That loss still affects us today. It
shaped the genetic makeup of all modern humans. Interestingly, this bottleneck
happened long before the famous out of Africa migration. This means African and
non-affrican populations were already starting to diverge even though they all
experienced this ancient genetic squeeze. During the early to middle pleaene
transition, a severe population bottleneck was detected in all 10 African
populations studied, but not in any of the 40 non-African populations. Even
though the severe bottleneck wasn't directly detected in non-African genomes,
it still affected their population history. After the bottleneck, the average
population size in non-African populations was about 20,000 while it was about
27,000 in African agriculturalist populations. A difference likely caused by
the hidden impact of the ancient bottleneck on non-African groups. Because
African populations did not leave the continent, their genetic lineages
preserved clearer signals of this early bottleneck event. In a focused analysis
of the Yoruba population in Nigeria, it was shown that even a small sample of
three individuals could detect the bottleneck. However, in non-African
populations, the signal was too weak to detect using standard methods.
Non-African populations also experienced the same severe bottleneck between
921,000 and 785,000 years ago, with their population shrinking to around 1,450
breeding individuals. similar to the results from African populations. This
confirms that the bottleneck was a shared and critical event in early human
history. To understand the severe population bottleneck better, researchers
simulated a gradual population decline starting 1.5 million years ago. The
population histories predicted by previous simulations were different from the
patterns seen in actual human genetic data, which suggests that the real
bottleneck was likely a sudden event rather than a slow decline. The bottleneck
lasted about 117,000 years and reduced the human population to roughly 1,280
individuals, comparable to current endangered species. This sharp decline
resulted in the loss of about 98.7% of ancestral humans and caused a
significant drop, about 65.85% in today's genetic diversity. It's possible the
population size during the bottleneck was even smaller than estimated due to
hidden subgroups and natural fluctuations, which may have also raised the risk
of inbreeding and extinction. Around 930,000 years ago, there was a major drop
in human population size, likely caused by drastic climate changes during the
early to middle pleaene transition, also called the 0.9 million years ago
event. During this time, glaciations became longer and harsher. Ocean
temperatures dropped to their lowest, and droughts and wildlife changes spread
across Africa and Eurasia. The early to middle pleaene transition which
occurred roughly between 1.2 million and 0.5 million years ago marks a profound
shift in earth's climate system. During this period the dominant pattern of
glacial interglacial cycles transformed significantly. Initially earth's
glacial cycles followed a relatively regular rhythm of about 41,000 years largely
influenced by variations in the earth's axial tilt oblquity. However, as the
early to middle pleaene transition progressed, this pattern gave way to more
irregular and extended cycles averaging 100,000 years in duration. This change,
despite no major alterations in orbital forcing, suggests the growing influence
of internal climate feedback mechanisms. One of the most notable climatic
changes during the early to middle pleaene transition was the increased
intensity and duration of glacial periods. Ice sheets, particularly in the
northern hemisphere, began to expand more extensively and persist for longer
durations. Glacials became much colder with greater volumes of ice and
significantly lower sea levels. In contrast, interglacial periods, although
warmer, were relatively brief. This resulted in a higher amplitude of climate
fluctuations and a marked asymmetry and glacial cycles, slow accumulation of
ice followed by rapid melting events. The climatic system thus became more
extreme and less predictable than in the preceding period. Several factors are
believed to have driven these changes. Although orbital variations continue to
influence climate, they alone do not explain the shift in periodicity. One key
hypothesis involves the dynamics of ice sheets. By the early to middle pleaene
transition, repeated glaciations had eroded much of the soft regalith, loose
soil and sediment under the ice, exposing bedrock. This made it easier for ice
sheets to grow thicker and remain stable for longer periods. In addition,
atmospheric carbon dioxide levels began to drop more significantly during
glacial periods, enhancing the Earth's cooling and reducing the climate systems
resilience. This marked a transition toward a climate system more heavily
influenced by internal feedbacks and threshold responses. The early to middle
pleaene transition also had wide-ranging environmental consequences across
different regions. In Africa, the climate became cooler and drier during glaci
leading to the contraction of forests and expansion of savas. These shifts are
thought to have played a role in hominin evolutionary pressures encouraging
dispersal, behavioral adaptation and technological development in Europe and
Asia. Expanding glaciers and cold step environments reshaped ecosystems and
migration patterns of both humans and animals. Meanwhile, marine sediment
records show increased ice rafted debris and stronger contrasts between glacial
and interglacial ocean temperatures, indicating broader disruptions in global
climate and ocean circulation. During the early to middle pleaene transition
about 1.2 to 0.8 million years ago, Africa's climate shifted from relatively
stable humid conditions to long dry spells punctuated by abrupt wet phases.
These fluctuations fragmented dense forests into isolated wooded patches and
expanded grasslands and savas across much of the continent, forcing hominins to
navigate a highly heterogeneous environment. Tectonic activity along the East
African rift system continued to reshape the landscape during this interval. As
rifting intensified, new lakes and river valleys formed in grab and basins,
creating corridors for animal and hominin movement, but also isolating
populations on uplifted rift flanks. This dynamic interplay of extension,
vcanism, and subsidance controlled local hydraology and vegetation patterns,
further driving habitat fragmentation. Animal communities responded to these
environmental changes by favoring open habitat specialists. Forest adapted
primates and other woodland species declined in many regions while grassland
adapted herbivores, antelopes, zebras, and grazing bovids became more abundant.
This fondal turnover documented in sediment cores from Lake Magatti coincided
with hamin adaptations such as increased mobility and endurance running enabling
early humans like Homo erectus to exploit savannah resources more effectively.
Stone tool traditions persisted and diversified across Africa during this
period. The Aulian handax industry, which originated around 1.7 million years
ago, continued to dominate but exhibited regional variations by 1 million years
ago. Toolkits included smaller, more refined hand axes and occasional pointed
implements, likely reflecting innovations in butchery and woodworking. Sites
such as olier show this mix of classic and modified acculent forms suggesting
both technological conservatism and experimentation. Hominin anatomy also
evolved gradually toward larger brain cases and more modern skull shapes.
Fossils dated toward the end of the transition display intermediate features
between homo erectus and later forms often grouped as homohidalbergensis. These
anatomical intermediates hint at a lineage leading toward homo sapiens, setting
the stage for the emergence of archaic and then anatomically modern humans.
Evidence for controlled use of fire and regular shelter use becomes clear in
this period. at Wonderwork Cave in South Africa micro stratographic analysis of
sediments dated to 1 million years ago has revealed in situ burned bone and
ashed plant remains demonstrating that early Aulian hominins were using fire
inside caves. Alongside this rock shelters across southern Africa show signs of
repeated occupation suggesting that fire and natural caves were integral to
homminin survival in cooler or drier microclimates. This ancient population
bottleneck might explain why very few hominin fossils from Africa and Eurasia
exist from 950 to 650,000 years ago. In Africa, only a handful of fossils from
this time have been found in places like Ethiopia and Algeria and they show similarities
to Homohidalbergensis. These fossils differ from homo anticcessor found in
Spain and East Asian fossils from this period belong to Homo erectus which
likely didn't contribute to modern human ancestry. Interestingly during this
bottleneck two ancestral chromosomes are believed to have fused into what is
now human chromosome 2 around 900 to 740,000 years ago. This period may also
mark a key speciation event that gave rise to the shared ancestors of modern
humans, Neanderthalss and Dennisovvens whose divergence happened around 765 to
550,000 years ago. After the bottleneck ended, the human population in Africa
rapidly grew about 20 times larger around 813,000 years ago. The use of fire
with early evidence from Israel around 790,000 years ago may have helped this
recovery. Climate improvements might have also played a role. In a new
analysis, researchers took a closer look at the claims and found a few
problems. To dig deeper, the researchers ran other population analysis tools to
track population changes to the same data. They didn't find any severe
bottleneck. So, what's the bottom line for all of us? This new analysis
suggests that its claim about a human population crash 1 million years ago
might not hold up. It's a reminder that in science, exciting ideas still need
to be carefully tested. And the simplest answer is often the best. If a
population bottleneck truly occurred 1 million years ago, we should see its
genetic signature in all human populations, including those outside Africa.
However, this signal is notably absent in non-African groups. The common
explanation that non-affrican populations lost these signals due to genetic
drift after migrating out of Africa does not hold up. Mathematical models show
that a bottleneck of that magnitude would have left a clear imprint across all
human DNA, making its absence in non-Africans a significant red flag. Next,
they looked at newer research that suggests humans didn't come from one single
ancient population. Instead, it looks like there were two or more human groups
living separately for a long time, which eventually came together and mixed.
One study suggests these groups split around 1.5 million years ago and came
back together about 300,000 years ago. One of those groups might have gone
through a bottleneck, but that's not the same event. Researchers have detected
a deep split in human ancestry around 1.5 million years ago, which rejoined
about 300,000 years ago, coinciding with the emergence of anatomically modern
humans. The ancestral split involved two major lineages, A and B. A ultimately
contributed about 80% of present-day human ancestry, undergoing a significant
bottleneck shortly after its formation. Lineage B contributed about 20% and
this introgression is shared by all living humans. While most introgress
material from lineage B seems to have been selected against certain segments
particularly those associated with neural development and processing appear to
have been retained suggesting adaptive value. The findings stand in contrast to
earlier models which also posited deep population structure but suggested
continuous gene flow between diverging lineages rather than complete isolation
followed by a later ad mixture. The new analysis supports a pulse model where A
and B remained isolated after their divergence rejoining around 300,000. The
evolutionary implications are profound. The two lineages A and B could
correspond to archaic homo species such as Homo erectus or Homohylebergensis.
Homo erectus appears in the fossil record from about 1.9 million to 0.8 million
years ago and was the first hominin to spread beyond Africa. Homohybergensis
lived from roughly 600,000 to 300,000 years ago and is often seen as the common
ancestor of both modern humans and Neanderthalss. It had a larger brain and a more
vertical forehead than homo erectus. So far, no ancient DNA from either species
has been definitively tied to the A or B genetic lineages, making a direct
fossil genome match still out of reach. The sharp bottleneck in lineage A may
reflect a founder event linked to a migration or ecological separation. These
findings raise further questions about the direction of gene flow between
modern humans and Neanderthalss or denisins and about which ancestral lineage
those archaic hominins were more closely related to. Connecting these genomic
insights to the fossil record remains a key challenge in human evolutionary
studies.