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When Humanity Was on the Brink – Just 1,000 Left
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May 4, 2025
A Brief History of Human Evolution and an Ancient Bottleneck Humans and chimpanzees share a common ancestor from over 6 million years ago. The genus Homo emerged around 2.8 million years ago in East Africa with species like Homo habilis and later Homo erectus, who spread into Eurasia and developed more advanced tools. Around 600,000 years ago, Homo heidelbergensis appeared, some of whom evolved into Neanderthals in Europe and modern Homo sapiens in Africa by about 300,000 years ago. Fossils from Morocco (Jebel Irhoud) mark some of the earliest Homo sapiens remains. Recent DNA analysis of modern humans suggests a dramatic population bottleneck occurred between 930,000 and 813,000 years ago, reducing the human population to about 1,280 individuals for over 100,000 years. This likely resulted from severe climate changes during the Early to Middle Pleistocene Transition, including intense glaciations, droughts, and ecological shifts in Africa. The bottleneck led to a major loss of genetic diversity and may explain the scarcity of fossils from this period. After it ended, human populations grew rapidly, possibly aided by better climate and the use of fire. However, some scientists challenge the bottleneck theory, arguing that non-African DNA doesn't show signs of it. Newer studies suggest human ancestry includes at least two ancient populations (Lineages A and B) that split around 1.5 million years ago and merged again about 300,000 years ago. Lineage A experienced a bottleneck, while B contributed a smaller but significant portion of modern human DNA. This complex picture reshapes our understanding of human origins, highlighting the role of climate, population mixing, and genetic change in our evolution.
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0:05
scientists believe we separated from our
0:07
closest relatives chimpanzees and
0:09
bonobos over 6 million years ago but the
0:13
species we call homo sapiens is thought
0:15
to have first appeared in Africa between
0:17
300,000 and 200,000 years ago since then
0:21
humans have spread all over the world
0:24
about 2.8 8 million years ago the first
0:27
members of our genus Homo appeared in
0:29
East Africa homohabilis one of the
0:32
earliest species lived from roughly 2.4
0:35
million years ago to 1.4 million years
0:38
ago and made simple old stone tools
0:41
small flakes and choppers for cutting
0:43
and pounding animal carcasses and plant
0:45
materials by about 2 million years ago a
0:48
descendant lineage gave rise to Homo
0:50
erectus in Africa the oldest securely
0:53
dated Homo erectus cranium from Dreolan
0:56
South Africa is about 2 million years
0:58
ago and by 1.8 million years ago these
1:02
populations had dispersed into Eurasia
1:04
for example Demoni in Georgia at 1.85
1:08
million years ago homo erectus showed a
1:11
leap in brain size fully modern body
1:13
proportions and a more sophisticated
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Aulian tool set including large bacial
1:19
handaxes over the next million years
1:21
Homo erectus diversified regionally
1:24
african forms sometimes called Homoaster
1:28
persisted alongside Asian populations
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like Java man and peing man well into
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the middle pleaene in Africa and Europe
1:37
populations gradually evolved larger
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brain cases and more rounded skulls
1:41
setting the stage for a new form by
1:43
about 600,000 years ago that new form
1:48
Homohylebergensis is recognized in
1:50
fossils dating between 600,000 and
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200,000 years ago most famously the Mau
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jaw in Germany and the Cobwway cranium
1:58
in Zambia 500 to 300,000 years ago
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homohidlebergensis populations built
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simple shelters used wooden and stone
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tipped spears for hunting and show
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evidence of fire use after about 400,000
2:12
years ago by about 400,000 years ago
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European homohy highlebergenses gave
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rise to the Neanderthalss while African
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homohylebergis lineages evolved into
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anatomically modern homo sapiens around
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300,000 years ago the earliest widely
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accepted Homo sapiens fossils those from
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Jebel Earhode Morocco are dated to
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300,000 years ago pushing back the
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origin of our species across much of
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Africa subsequent dispersals and
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episodes of interbreeding with
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Neanderthalss and Dennisovvens would
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further shape the genetic tapestry of
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all later humans using DNA from people
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living today scientists have learned a
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lot about recent human history how our
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ancestors spread across different
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continents for example but we still
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don't know much about what happened long
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before modern humans appeared during the
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earlier phases of the human lineage
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especially in the pletosene period which
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lasted from about 2.5 million to 12,000
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years ago this early history is
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important if we want to fully understand
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where we came from one of the biggest
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challenges is that ancient DNA from
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Africa older than homo sapiens is
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extremely rare or missing because it
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doesn't preserve well in hot climates so
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instead of ancient DNA scientists try to
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look for clues in the DNA of people
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living today the DNA of modern humans
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still carries signals of population
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changes from the distant past one of the
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ways scientists study this is through
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something called the site frequency
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spectrum a tool that looks at how common
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or rare different mutations are in the
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DNA of a population these patterns can
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tell us how big or small populations
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were in the past and when they may have
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gone through big changes like sudden
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shrinkage or growth recently scientists
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used advanced mathematical modeling on
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DNA from today's populations to figure
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out what likely happened in the past
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they uncovered a shocking chapter in our
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history a time when the human population
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dropped to dangerously low numbers this
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research shows that our ancestors went
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through a major population bottleneck in
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the past reducing the number of breeding
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individuals to just around a thousand
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that's an incredibly small number so
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small it nearly led to extinction they
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studied change in population size over
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time by analyzing patterns in modern DNA
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the method was applied to DNA from 10
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African and 40 non-African populations
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focusing on parts of the genome that
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aren't influenced much by natural
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selection or sequencing errors that loss
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still affects us today it shaped the
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genetic makeup of all modern humans
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interestingly this bottleneck happened
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long before the famous out of Africa
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migration this means African and
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non-affrican populations were already
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starting to diverge even though they all
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experienced this ancient genetic squeeze
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during the early to middle pleaene
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transition a severe population
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bottleneck was detected in all 10
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African populations studied but not in
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any of the 40 non-African populations
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even though the severe bottleneck wasn't
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directly detected in non-African genomes
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it still affected their population
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history after the bottleneck the average
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population size in non-African
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populations was about 20,000 while it
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was about 27,000 in African
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agriculturalist populations a difference
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likely caused by the hidden impact of
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the ancient bottleneck on non-African
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groups because African populations did
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not leave the continent their genetic
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lineages preserved clearer signals of
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this early bottleneck event in a focused
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analysis of the Yoruba population in
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Nigeria it was shown that even a small
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sample of three individuals could detect
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the bottleneck however in non-African
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populations the signal was too weak to
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detect using standard methods
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non-african populations also experienced
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the same severe bottleneck between
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921,000 and
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785,000 years ago with their population
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shrinking to around
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1,450 breeding individuals similar to
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the results from African populations
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this confirms that the bottleneck was a
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shared and critical event in early human
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history to understand the severe
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population bottleneck better researchers
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simulated a gradual population decline
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starting 1.5 million years ago the
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population histories predicted by
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previous simulations were different from
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the patterns seen in actual human
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genetic data which suggests that the
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real bottleneck was likely a sudden
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event rather than a slow decline the
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bottleneck lasted about 117,000 years
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and reduced the human population to
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roughly
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1,280 individuals comparable to current
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endangered species this sharp decline
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resulted in the loss of about
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98.7% of ancestral humans and caused a
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significant drop about
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65.85% in today's genetic diversity it's
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possible the population size during the
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bottleneck was even smaller than
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estimated due to hidden subgroups and
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natural fluctuations which may have also
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raised the risk of inbreeding and
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extinction around 930,000 years ago
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there was a major drop in human
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population size likely caused by drastic
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climate changes during the early to
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middle pleaene transition also called
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the 0.9 million years ago event during
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this time glaciations became longer and
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harsher ocean temperatures dropped to
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their lowest and droughts and wildlife
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changes spread across Africa and Eurasia
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the early to middle pleaene transition
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which occurred roughly between 1.2
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million and 0.5 million years ago marks
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a profound shift in earth's climate
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system during this period the dominant
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pattern of glacial interglacial cycles
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transformed significantly initially
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earth's glacial cycles followed a
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relatively regular rhythm of about
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41,000 years largely influenced by
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variations in the earth's axial tilt
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oblquity however as the early to middle
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pleaene transition progressed this
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pattern gave way to more irregular and
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extended cycles averaging 100,000 years
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in duration this change despite no major
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alterations in orbital forcing suggests
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the growing influence of internal
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climate feedback mechanisms one of the
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most notable climatic changes during the
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early to middle pleaene transition was
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the increased intensity and duration of
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glacial periods ice sheets particularly
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in the northern hemisphere began to
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expand more extensively and persist for
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longer durations glacials became much
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colder with greater volumes of ice and
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significantly lower sea levels in
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contrast interglacial periods although
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warmer were relatively brief this
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resulted in a higher amplitude of
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climate fluctuations and a marked
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asymmetry and glacial cycles slow
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accumulation of ice followed by rapid
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melting events the climatic system thus
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became more extreme and less predictable
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than in the preceding period several
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factors are believed to have driven
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these changes although orbital
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variations continue to influence climate
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they alone do not explain the shift in
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periodicity one key hypothesis involves
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the dynamics of ice sheets by the early
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to middle pleaene transition repeated
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glaciations had eroded much of the soft
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regalith loose soil and sediment under
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the ice exposing bedrock this made it
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easier for ice sheets to grow thicker
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and remain stable for longer periods in
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addition atmospheric carbon dioxide
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levels began to drop more significantly
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during glacial periods enhancing the
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Earth's cooling and reducing the climate
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systems resilience this marked a
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transition toward a climate system more
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heavily influenced by internal feedbacks
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and threshold responses the early to
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middle pleaene transition also had
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wide-ranging environmental consequences
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across different regions in Africa the
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climate became cooler and drier during
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glaci leading to the contraction of
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forests and expansion of savas these
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shifts are thought to have played a role
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in hominin evolutionary pressures
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encouraging dispersal behavioral
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adaptation and technological development
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in Europe and Asia expanding glaciers
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and cold step environments reshaped
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ecosystems and migration patterns of
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both humans and animals meanwhile marine
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sediment records show increased ice
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rafted debris and stronger contrasts
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between glacial and interglacial ocean
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temperatures indicating broader
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disruptions in global climate and ocean
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circulation during the early to middle
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pleaene transition about 1.2 to 0.8
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million years ago Africa's climate
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shifted from relatively stable humid
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conditions to long dry spells punctuated
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by abrupt wet phases these fluctuations
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fragmented dense forests into isolated
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wooded patches and expanded grasslands
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and savas across much of the continent
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forcing hominins to navigate a highly
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heterogeneous environment tectonic
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activity along the East African rift
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system continued to reshape the
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landscape during this interval as
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rifting intensified new lakes and river
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valleys formed in grab and basins
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creating corridors for animal and
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hominin movement but also isolating
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populations on uplifted rift flanks this
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dynamic interplay of extension vcanism
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and subsidance controlled local
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hydraology and vegetation patterns
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further driving habitat fragmentation
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animal communities responded to these
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environmental changes by favoring open
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habitat specialists forest adapted
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primates and other woodland species
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declined in many regions while grassland
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adapted herbivores antelopes zebras and
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grazing bovids became more abundant this
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fondal turnover documented in sediment
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cores from Lake Magatti coincided with
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hamin adaptations such as increased
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mobility and endurance running enabling
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early humans like Homo erectus to
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exploit savannah resources more
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effectively stone tool traditions
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persisted and diversified across Africa
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during this period the Aulian handax
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industry which originated around 1.7
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million years ago continued to dominate
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but exhibited regional variations by 1
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million years ago toolkits included
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smaller more refined hand axes and
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occasional pointed implements likely
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reflecting innovations in butchery and
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woodworking sites such as
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olier show this mix of classic and
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modified acculent forms suggesting both
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technological conservatism and
12:52
experimentation hominin anatomy also
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evolved gradually toward larger brain
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cases and more modern skull shapes
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fossils dated toward the end of the
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transition display intermediate features
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between homo erectus and later forms
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often grouped as
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homohidalbergensis these anatomical
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intermediates hint at a lineage leading
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toward homo sapiens setting the stage
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for the emergence of archaic and then
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anatomically modern humans evidence for
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controlled use of fire and regular
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shelter use becomes clear in this period
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at Wonderwork Cave in South Africa micro
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stratographic analysis of sediments
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dated to 1 million years ago has
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revealed in situ burned bone and ashed
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plant remains demonstrating that early
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Aulian hominins were using fire inside
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caves alongside this rock shelters
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across southern Africa show signs of
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repeated occupation suggesting that fire
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and natural caves were integral to
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homminin survival in cooler or drier
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microclimates
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this ancient population bottleneck might
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explain why very few hominin fossils
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from Africa and Eurasia exist from 950
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to 650,000 years ago in Africa only a
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handful of fossils from this time have
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been found in places like Ethiopia and
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Algeria and they show similarities to
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Homohidalbergensis
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these fossils differ from homo
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anticcessor found in Spain and East
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Asian fossils from this period belong to
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Homo erectus which likely didn't
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contribute to modern human ancestry
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interestingly during this bottleneck two
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ancestral chromosomes are believed to
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have fused into what is now human
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chromosome 2 around 900 to 740,000 years
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ago this period may also mark a key
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speciation event that gave rise to the
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shared ancestors of modern humans
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Neanderthalss and Dennisovvens whose
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divergence happened around 765 to
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550,000 years ago after the bottleneck
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ended the human population in Africa
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rapidly grew about 20 times larger
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around
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813,000 years ago the use of fire with
15:01
early evidence from Israel around
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790,000 years ago may have helped this
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recovery climate improvements might have
15:08
also played a role in a new analysis
15:11
researchers took a closer look at the
15:13
claims and found a few problems to dig
15:16
deeper the researchers ran other
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population analysis tools to track
15:20
population changes to the same data they
15:22
didn't find any severe bottleneck so
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what's the bottom line for all of us
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this new analysis suggests that its
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claim about a human population crash 1
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million years ago might not hold up it's
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a reminder that in science exciting
15:37
ideas still need to be carefully tested
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and the simplest answer is often the
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best if a population bottleneck truly
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occurred 1 million years ago we should
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see its genetic signature in all human
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populations including those outside
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Africa however this signal is notably
15:53
absent in non-African groups the common
15:56
explanation that non-affrican
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populations lost these signals due to
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genetic drift after migrating out of
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Africa does not hold up mathematical
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models show that a bottleneck of that
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magnitude would have left a clear
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imprint across all human DNA making its
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absence in non-Africans a significant
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red flag next they looked at newer
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research that suggests humans didn't
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come from one single ancient population
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instead it looks like there were two or
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more human groups living separately for
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a long time which eventually came
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together and mixed one study suggests
16:29
these groups split around 1.5 million
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years ago and came back together about
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300,000 years ago one of those groups
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might have gone through a bottleneck but
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that's not the same event researchers
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have detected a deep split in human
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ancestry around 1.5 million years ago
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which rejoined about 300,000 years ago
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coinciding with the emergence of
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anatomically modern humans the ancestral
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split involved two major lineages A and
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B a ultimately contributed about 80% of
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present-day human ancestry undergoing a
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significant bottleneck shortly after its
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formation lineage B contributed about
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20% and this introgression is shared by
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all living humans while most introgress
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material from lineage B seems to have
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been selected against certain segments
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particularly those associated with
17:22
neural development and processing appear
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to have been retained suggesting
17:26
adaptive value the findings stand in
17:29
contrast to earlier models which also
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posited deep population structure but
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suggested continuous gene flow between
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diverging lineages rather than complete
17:38
isolation followed by a later ad mixture
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the new analysis supports a pulse model
17:43
where A and B remained isolated after
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their divergence rejoining around
17:48
300,000 the evolutionary implications
17:50
are profound the two lineages A and B
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could correspond to archaic homo species
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such as Homo erectus or
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Homohylebergensis homo erectus appears
18:01
in the fossil record from about 1.9
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million to 0.8 million years ago and was
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the first hominin to spread beyond
18:09
Africa homohybergensis lived from
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roughly 600,000 to 300,000 years ago and
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is often seen as the common ancestor of
18:18
both modern humans and Neanderthalss it
18:21
had a larger brain and a more vertical
18:23
forehead than homo erectus so far no
18:26
ancient DNA from either species has been
18:28
definitively tied to the A or B genetic
18:30
lineages making a direct fossil genome
18:33
match still out of reach the sharp
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bottleneck in lineage A may reflect a
18:38
founder event linked to a migration or
18:40
ecological separation these findings
18:43
raise further questions about the
18:44
direction of gene flow between modern
18:47
humans and Neanderthalss or denisins and
18:50
about which ancestral lineage those
18:52
archaic hominins were more closely
18:54
related to connecting these genomic
18:56
insights to the fossil record remains a
18:58
key challenge in human evolutionary
19:00
studies
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