ආසියාව ද අප්‍රිකාව ද?
මේ පසුගිය 7වැනි සෙනසුරාදා නිව් සයන්ටිස්ට් සඟරාවේ
පළ වූ ලිපියක්.
මිනිස් ජීවියා ඇතිවුණේ ආසියාවේ ද අප්‍රිකාවේ ද යන්න ප්‍රශ්නයක් වෙලා. මා මෙරට ඊනියා විද්‍යාඥයන්ගෙන්
මෙයට විසඳුමක් බලාපොරොත්තු වන්නේ නැහැ. එහෙත් මගේ ප්‍රශ්නයක් තියෙනවා. මිනිසා එක
තැනක ඇතිවුණේ ඇයි ද යන්න. මිනිසා කිහිප
තැනක ඇතිවන්නට තිබෙන බාධාව කුමක් ද? ඇතැම් විද්‍යාඥයන්ටත් ඒ ප්‍රශ්නය තියෙනවා. එහෙත් නිල බටහිර විද්‍යාව
එය ඩී එන් ඒ කතා කියා මහහරිනවා. මෙයට පෙර අප්‍රිකා කතාව කී වෙලේ මා බහුසම්භව ප්‍රශ්නය
නිව්සයන්ටිස්ට් කතුවරයාගෙන් ඇසුවා. ඔහු එයට දුන් පිළිතුරත් මේ සමග ඇති.
මා වඩාත් භයානක ප්‍රශ්නය ඇසුවේ නැහැ. සත්ව පරිණාමය ඩාවින් කියන
ආකාරයෙන් ම සිදුවිය යුතු ද? එය තවත් කතාවක් පමණක් නොවේ ද?
Asia’s mysterious role in the
early origins of humanity
Bizarre fossils from China are revealing our
species' Asian origins and rewriting the story of human evolution
By Kate
Douglas
DECEMBER 1941. Japan has
just entered the second world war. China, already fighting its neighbour, is in
the firing line. At the Peking Union Medical College Hospital, Hu Chengzhi
carefully packs two wooden crates with the world’s most precious
anthropological artefacts. Peking Man – in reality some 200 fossilised teeth
and bones, including six skulls – is to be shipped to the US for safekeeping.
This is the last anyone ever sees of him.
At the time, the Peking Man remains were the oldest known
fossils belonging to human ancestors. Their discovery in the 1920s and 30s
caused a sensation, triggering declarations that the cradle of humanity had
been found. But just a few decades later, all eyes had turned to Africa. A slew
of discoveries there left little doubt that it was our true ancestral home. As
far as human evolution was concerned, Asia was out of the picture.
Not any more. The last decade has seen the discovery of new
Asian fossils, among others by Chinese palaeoanthropologists with a renewed
interest in their heritage. As key moments in our past are rewritten, the
spotlight is once more turning east.
The first Peking Man
remains were found in 1923, nearly 50 kilometres outside Beijing. Alongside the
broad-nosed individuals with thick brows were burnt animal bones, suggesting an
early human ancestor capable of controlling fire. Only four other ancestral
human species had been discovered at that time, including Neanderthals in
Germany and Australopithecus africanus, identified from the
ape-like Taung Child remains in South Africa. Team leader Davidson Black
believed the Chinese fossils represented a new species, which he called Sinanthropus pekinensis.
For a while, all the
excitement in palaeoanthropology focused on east Asia. Then, in the 1950s,
husband-and-wife team Louis and Mary Leakey began digging at Olduvai gorge in
Tanzania. By 1959, they had discovered a 1.8-million-year-old species, Paranthropus boisei. This presaged a flood of
remarkable discoveries in East Africa, including the earliest Homo species – Homo habilis – at
Olduvai; footprints at nearby Laetoli, revealing that our ancestors walked
upright at least 3.7 million years ago; and the famous “Lucy” (Australopithecus afarensis), that lived 3.2 million
years ago, in Ethiopia. Peking Man and Asia were sidelined.
From these and
subsequent discoveries emerged the standard model of human evolution. It traces
our family tree back to a split with the forebears of chimps 6 to 10 million years
ago. The
next few million years saw the evolution of a profusion of hominin species the
length and breadth of Africa, before our own genus, Homo,
emerged around 2 million years ago.
H. erectus, an early member of our genus, clearly
had a wanderlust. It migrated all the way to South-East Asia as early as 1.8
million years ago. Today, Peking Man is recognised as a late representative
that lived 700,000 years ago. While nobody disputes that several
human species populated Eurasia very early on, the textbook version sees them
as evolutionary dead ends. Our own species, the story goes, descends directly
from African H. erectus and only emerged from the continent some
60,000 years ago, at which point it swept across the globe, replacing all other
hominin species.
That, until very recently, was the accepted story. There were
details to fill out, but the plot and main characters were clear. As fossils
trickled out of Asia, drawing far less attention in the West than African
fossils did, they were often dismissed because they contradicted the dominant
narrative.
Transitional humans
In 1992, for instance,
researchers reported finding a pair of 900,000-year-old skulls in Yunxian, central China. Their
features looked like a mix of H. erectus and H. sapiens – which was odd since H. sapiens should theoretically have been firmly
sequestered in Africa at the time. “Dali Man”, a 260,000-year-old skull found
14 years earlier in the central province of Shaanxi, had a similar mix of
features, typical of “transitional forms”, that cannot be ascribed to any well-defined
species. Although the Yunxian and Dali Man fossils are particularly fine
examples, many more have been found in east Asia.
Then, in 2009, Chinese
scientists announced the discovery of a 110,000-year-old jawbone in the
southern province of Guangxi. Though relatively primitive, it displayed a prominent human-like chin. The team classified it as H. sapiens, which would mean that our species was in
Asia a good 50,000 years before we previously thought. Still, many
were sceptical, reluctant to rewrite humanity’s origins. Some suggested it may
have been a hybrid of H. sapiens with another now-extinct
species – though that would still imply that humans were in east Asia. Who or
what the Guangxi remains belonged to is
still hotly debated.
The tide began to turn
in 2015, when 47 teeth were found inside a cave in Daoxian, also in southern
China. Teeth are one of the best ways to distinguish between hominin species,
and these were distinctly human – belonging to our own species – not to mention
very old. According to Wu Liu at the Institute of Vertebrate Paleontology and
Paleoanthropology in Beijing and his colleagues, they
had been lying around for 80,000 to 120,000 years. For Wu, the mounting evidence could
only mean one thing: “Early modern humans were in southern China at least
100,000 years ago.”
That would put our species’ first foray out of Africa at least
40,000 years further back in time, yet this early Asian expansion hypothesis is
gaining traction. “The traditional view, that modern humans swept out of Africa
as a single exodus 60,000 years ago is now being called into question,” says
Michael Petraglia at the Max Planck Institute for the Science of Human History,
Germany.
In December 2017,
Petraglia co-authored a review that considers the “Asian perspective” on human
evolution, drawing on all the evidence. “Palaeoanthropologists are now
increasingly arguing, on the basis of fossil, archaeological and genetic
evidence, that humans began spreading out of Africa by at least 120,000 years
ago, and in multiple waves,” says Petraglia.
An earlier exit from
Africa fits better with other recent discoveries. Just last year, a collection
of Moroccan fossils suggested that our species could be 300,000 to 350,000
years old, adding at least 100,000 years to our history. Then, earlier this
year, we learned that a group of H. sapiens was living in what is now Israel at
least 177,000 years ago. Another group was making tools in south India at around the same time. And in April, reports of a fossilised
finger bone pointed to the presence of H. sapiens in what is now
Saudi Arabia
at least 85,000 years ago. Early humans clearly weren’t the stay-at-homes we
once thought. There seems little doubt that our direct ancestors ventured east
out of Africa far earlier than the standard narrative allows.
This is a huge change to
the standard view.But it doesn’t explain those weird transitional fossils from
China that display a mix of H. erectus and H. sapiens features. Being hundreds of thousands of
years old, they predate even the earlier exodus out of Africa. The most radical
suggestion is that they are evidence that H. erectus evolved
into H. sapiens in east Asia. Wu Xinzhi at the Chinese
Academy of Sciences in Beijing is the fiercest proponent of this suggestion, a
form of “multiregionalism”. The idea that we evolved from a number of separate
populations was once regarded as maverick, but has become more respectable.
Most notably, the latest evidence from inside Africa undermines the notion that
H. sapiens emerged from a single population in the east of the continent. Instead, most now agree there were
isolated populations across Africa that sporadically came together and mated, creating a variety of human forms.
Wu Xinzhi’s suggestion,
however, remains radical. He believes that the transitional fossils are evidence of Asian H. erectus evolving into our own species in Asia, that
Peking Man was an ancestor of modern Asian people and that Asia should have
equal billing with Africa as the birthplace of our species. While some Chinese
palaeontologists support this view, others see a hint of nationalism at play.
To bolster his argument that China’s hominins were evolving along similar lines
to those in Africa, Wu Xinzhi, along with Sheela Athreya at Texas A&M
University, recently showed that the Dali skull has many features in common with the early H. sapiens fossils
from Morocco.
The genetic evidence, however, still points to a common African origin: per cent
of the DNA in Chinese populations can be traced back to Africa.
A more likely explanation
for the transitional fossils is that Asia, like Africa, was once home to
various human groups that exchanged genes. Along with H. erectus, genetic studies have revealed a number of hominins
whose identity is still a mystery. Later, there was the diminutive “hobbit”, H. floresiensis,
found on the Indonesian island of Flores from 100,000 years ago, preceded by an
ancestral species going back 700,000 years. Denisovans were probably there too: although their identified remains
currently amount to just three teeth and a finger bone found in Siberia, dental
and genetic evidence indicate they were also in South-East Asia. Even Neanderthals, which have only
been positively identified as far east as central Asia’s Altai mountains, may once have roamed further east. Their characteristic features are now being identified in Chinese fossils.
If these various species
interbred, they should have left behind hybrids scattered across Asia (see “Hybrid shapes”). Indeed, the surprisingly rich
variety among east Asian fossils suggests hybridisation was widespread, says
María Martinón-Torres, director of the National Research Centre on Human
Evolution in Spain.
Martinón-Torres and her
colleagues have also shown that hominins in Africa and Eurasia did evolve
relatively independently for a long time. When they examined 5000 fossil teeth
spanning 2.5 million years, they found that each
continent had its own distinct type of teeth – strong evidence that Eurasia was a
centre of speciation in its own right.
Rather than
multiregionalism, she and her collaborators suggest a
“source and sink” model to explain the human settlement of east Asia. They believe the
huge variety of fossils points to repeated colonisation, interbreeding and
extinctions, with populations thriving and disappearing depending on
fluctuations in the climate over hundreds of thousands of years. During cold
periods, much of central Asia and the northern steppe would have become
uninhabitable. These are the “sinks”. But hominins would have been able to
survive in more southerly regions, on some islands and in regions where the climate
remained relatively stable, such as the Middle East – the “sources”.
If correct, this points
to a strong but discontinuous occupation of east Asia. Although Martinón-Torres
doesn’t believe that our species arose independently in Asia, she does think it
likely that we have roots in Asia – though probably not in the Far East. Peking
Man was a true pioneer, but most of his relatives didn’t travel nearly as far,
settling instead in the Middle East where the climate was more favourable.
This, she suggests, was the source population of Neanderthals, Denisovans and
another branch of our family tree, which migrated back to Africa before
evolving there into H. sapiens. “Maybe Africa was not
the only human cradle,” she says.
The truth is, the story of our evolution is still being
rewritten and we can’t be sure how it will turn out. What is certain is that
Asia can no longer be sidelined. It is possible that the species we evolved
from made its own migration into Africa before giving rise to us. Our ancestors
then left Africa at least 100,000 years ago and travelled the breadth of
Eurasia for millennia.
The Chinese government
recently set up a lab at the Institute of Vertebrate Paleontology and
Paleoanthropology to extract and sequence ancient genetic material. “We only
have one genome,” says Qiaomei Fu, who heads the lab. It is from a
40,000-year-old individual found near Beijing. More DNA work should shed new light,
perhaps even identify new human species. These and other future discoveries in
Asia are sure to unearth more twists in the tale. “Asia,” says Martinón-Torres,
“is like Pandora’s box.” Which just makes it more enticing.
How to spot a hybrid fossil
We now know that our ancestors, in all their forms, were
a promiscuous bunch. It seems as if every new genetic study reveals yet more
interbreeding between groups that were once thought to be distinct species. But
in the medley of bones being dug up, how do we identify which ones are from
hybrids? Studies of living primates offer useful insights.
Contrary to expectations, hybrids aren’t a
mosaic of their parents’ features. Instead, interbreeding in baboon species often
leads to evolutionary innovation. The offspring tend to take highly variable forms, be
unusually large or small, and have overcrowded and misaligned teeth.
This has led María Martinón-Torres,
director of the National Research Centre on Human Evolution in Spain, and her
colleagues in China to suggest a new explanation for the many strange human forms that have been found on
the Indonesian island of Flores. Perhaps, they say, the hobbit species, H. floresiensis, hybridised with H. sapiens or possibly even H.
erectus. “Hence its considerable number of pathologies and unusual
features,” says Martinón-Torres.
How to spot a hybrid fossil
We now know
that our ancestors, in all their forms, were a promiscuous bunch. It seems as
if every new genetic study reveals yet more interbreeding between groups that
were once thought to be distinct species. But in the medley of bones being dug
up, how do we identify which ones are from hybrids? Studies of living primates
offer useful insights.
Contrary to expectations, hybrids aren’t a
mosaic of their parents’ features. Instead, interbreeding in baboon species often
leads to evolutionary innovation. The offspring tend to take highly variable forms, be
unusually large or small, and have overcrowde
d and misaligned teeth.
This has led María Martinón-Torres,
director of the National Research Centre on Human Evolution in Spain, and her
colleagues in China to suggest a new explanation for the many strange human forms that have been found on
the Indonesian island of Flores. Perhaps, they say, the hobbit species, H. floresiensis, hybridised with H. sapiens or possibly even H.
erectus. “Hence its considerable number of pathologies and unusual
features,” says Martinón-Torres.
This article appeared in print
under the headline “Our African Origins”
A shorter version of this article was
published in New Scientist magazine on 30 June 2018
Magazine issue 3185,
published 7 July 2018
Dear
Nalin de Silva –
It
seems to me that there’s more an assumption that there’s one cradle for the
life we see now with its ubiquitous DNA coding…
On
the question of human origins, have you seen the feature in the issue dated
tomorrow: https://www.newscientist.com/article/mg23931850-200-asias-mysterious-role-in-the-early-origins-of-humanity/
(I
had to hold off responding until the ink dried on that… sorry.)
Mike
Holderness
Readers’ editor
Readers’ editor