Author : Wahid Ahmad
Early modern humans
departing Africa likely made their initial stop in India, where the first
diversifications of mitochondrial and Y chromosome lineages outside Africa
occurred. From India, one group moved eastward to Southeast Asia. This coastal
migration facilitated the exploration of the lost continents of Sunda and
Sahul.
During periods of
low sea levels in the past, Southeast Asian regions, including islands like
Sumatra, Java, Bali, and Borneo, formed an extended landmass known as Sundaland
or Sunda. This landmass was linked by substantial land bridges during the last
glacial maximum, around twenty thousand years ago.
Over time,
Sundaland experienced sea-level rises resulting in a sea-level increase of one
hundred and twenty meters that shaped the present-day geographical layout of
Southeast Asia. Sahul comprised regions like Australia, New Guinea, and
Tasmania, connected during the last glacial maximum due to lower sea levels.
Named after British
naturalist Alfred Russel Wallace, Wallacean is a transitional biogeographic
region located between Sunda and Sahul. It is bounded by two key biogeographic
lines: Wallace’s Line to the west and Lydekker’s Line to the east.
Wallacean comprises
a collection of islands, including Sulawesi, Lombok, Flores, Timor, and the
Moluccas, and is characterized by its distinct flora, fauna, and geological
history.
The Wallacean
islands contain a mix of animals from both regions, including marsupials and
placental mammals, each island having its own unique combination of species. To
the west of Wallace’s Line, ecosystems are dominated by placental mammals,
while to the east near Australasia, ecosystems are mainly marsupial-dominated.
The separation of
the Sunda and Sahul landmasses by sixty miles of Wallace’s Line led to the
independent evolution of Asian and Australian fauna. Asia developed placental
mammals, while Sahul hosted marsupials.
The island of
Flores revealed evidence of Stegodon elephants and Homo floresiensis. Besides
Homo floresiensis, Southeast Asia’s human habitation history of nearly two
million years includes Homo erectus and potentially Denisovans. Anatomically
modern humans reached Australia through ancient seafaring, supported by
archaeological finds.
The earliest human
occupation on Sahul offers indirect evidence of advanced seafaring skills.
Crossing the 60-mile-wide channel between Sunda and Sahul required intentional
voyaging and likely guidance from indicators like forest fires or bird
migrations.
Around 50,000 to
65,000 years ago, modern humans spread rapidly through Southeast Asia due to
lower sea levels. They crossed Wallacea's islands to reach Sahul.
The necessity of
intentional voyaging is highlighted by the absence of archaic humans and large
mammals on Sahul, suggesting directed efforts. This migration from Sunda
to Sahul was a remarkable feat for several reasons. Firstly, it required
advanced maritime skills to navigate the waters between islands.
Secondly, the
migrants had to adapt to new environments, including unfamiliar plants and animals,
as they settled on each new island or continent. Lastly, this migration
occurred tens of thousands of years before the settlement of the Americas,
making it an early and significant human movement.
Considering factors like island
distances, terrain, and sea crossings, two main migration routes have been
suggested for the migration of early humans from the Sunda region to Sahul: the
northern route and the southern route. The northern route followed the northern
part of Wallacea. It was preferred due to shorter distances between islands and
better visibility.
Around sixty-five thousand years
ago, during lower sea levels, the Sunda coastline extended along Borneo's
eastern side, reaching Java and Bali before curving west. The route likely
began near the Balabalagan islands and extended to Misool Island. Potential
paths included islands like Obi, Kofiau, Buru, Seram, Peleng, and Sula. This
route involved three crossings after Sulawesi, each taking two to three days,
with the destination island visible throughout the journey. The southern route
involved longer, riskier voyages between Timor and Australia, requiring
well-planned sea crossings. Islands on Sahul Banks were visible from high
points on Timor and Roti.
Around seventy thousand years
ago, due to higher sea levels, the Sunda coastline was different. Another route
through the Nusa Tenggara archipelago was possible, starting from the southern
tip of Sumatra, passing Timor, and reaching Sahul. The successful settlement of
Sahul likely involved intentional, coordinated voyages by distinct populations
over centuries.
They might have followed ocean
currents and wind patterns. The southern route's past savanna corridor aided
the migration of people adapted to such environments. After reaching Sahul,
colonists occupied Australian deserts, and Indic sites dating back to fifty to
forty-five thousand years ago. The northern route's environment resembled New
Guinea, requiring coastal or forest travel.
Initial arrival
into Sahul included two groups within fifty to sixty-five thousand years. One
settled in Northern Sahul, which includes New Guinea and Near Oceania, and the
other in Southern Sahul, which includes Australia and nearby islands. The two
groups remained in relative isolation in the postglacial period after thirty
thousand years ago, causing further diversification of human lineages in the
region.
In Australia,
evidence suggests that humans arrived around fifty to sixty thousand years ago.
The Madjedbebe rock shelters provide some of the earliest proof of human
presence, with artifacts dating back to that time. Skeletal remains from Lake
Mungo display modern physical features, and ancient DNA from these remains
reflects a unique Aboriginal lineage that still exists in today's population.
Around forty-five
thousand years ago, people began colonizing Australia, adapting to diverse
environments like deserts, grasslands, and coasts. They expanded gradually,
leading to the unique biological and cultural diversity that characterizes
Australia. In northeast Australia around forty-one thousand years ago, a
decline in large animals and increased fires occurred, likely due to human
hunting amid drying conditions. As people spread across the region, they relied
on available animals.
They used various
strategies to hunt small to medium-sized game and utilized permanent desert
lakes for resources. Fishing tools like nets and spears, along with plant foods
like yams, were used.
The Denisovans were
an ancient human population that once roamed the Earth alongside the
Neanderthals and modern Homo sapiens. However, nobody knew about them until a
groundbreaking discovery in the Denisovan Cave in Siberia.
In two thousand and
ten, an analysis of mitochondrial DNA extracted from a finger bone found in the
Denisova Cave in the Russian Altai revealed the existence of a new hominin
taxon. This newly described group was found to be genetically distinct from
both Homo sapiens and Homo neanderthalensis. The mitochondrial DNA sequences
showed haplotypes outside the range of variation of modern humans and
Neanderthals.
Outside of Denisova
Cave, a mandible found in Baishiya Cave in China was tentatively attributed to
Denisovans based on proteomic evidence. Sediment DNA extracted from the site
also confirmed the presence of Denisovans. These limited fossil findings
suggest that Denisovans may have been widespread across continental Asia,
island Southeast Asia, and Near Oceania.
During the Late
Middle to Early Upper Pleistocene, three distinct groups emerged: early modern
humans in Africa, Neanderthals in Europe, and Denisovans in Asia. Molecular
data suggests that the split between Neanderthals and Denisovans occurred
between three hundred and eighty to four hundred and seventy thousand years
ago, while the branch leading to Denisovans and modern humans diverged around
eight hundred thousand years ago. Homo heidelbergensis migrated to Eurasia
around eight hundred thousand years ago, as indicated by the sites in Israel in
the Levant region.
Around four hundred
and fifty thousand to three hundred and fifty thousand years ago, Homo
heidelbergensis from the Levant migrated to the Iranian Plateau and further to
Central and North Asia, including Denisova Cave in southern Siberia, giving
rise to Denisovans. This migration marked the splitting of late Homo
heidelbergensis into Neanderthals and Denisovans.
Moreover, late Homo
heidelbergensis from the Levant also reached to regions such as Turkmenia,
Kazakhstan, and Mongolia, as indicated by the appearance of Acheulean
industries in those areas. Their likely route was north of the Himalayas and
Tibet. The mandible found in Baishiya Cave on the northeastern Tibetan Plateau
dates back to at least one hundred and sixty thousand years ago.
The presence of Denisovans in the
region is also supported by the adaptation of Tibetans to high altitudes
possibly through introgression from Denisovans.
Hybridization events occurred
between these groups, suggesting a meta population that includes all three
taxa. The Altai region, where Denisova Cave is located, was occupied by
Neanderthals, Denisovans, and modern humans, although the precise timing and
coexistence of these groups remain uncertain. The Denisova 11 girl represents
an example of hybridization, with a Neanderthal mother and a Denisovan father.
The study of ancient DNA has
revealed significant genetic differences between Denisovans and modern humans.
Denisovan genomes differ from the standard human genome by 11.7 per cent, while
the difference between Neanderthals and modern humans is 12.2 per cent.
Denisovans and Neanderthals are closely related but distinct from Homo sapiens.
Denisovans show higher genetic
diversity than Neanderthals but lower diversity than modern humans, indicating
a larger and more diverse population. Their geographic distribution ranged from
North Asia to Southeast Asia.
Denisovans carried genetic traits
associated with dark skin, brown hair, and brown eyes, which are also present
in modern humans.
Denisovans likely played a role
in adaptive introgression, where early Homo sapiens acquired genes from
Denisovans that enhanced adaptation, disease resistance, and immune systems.
The presence of human leukocyte antigens in modern humans, inherited from
Denisovans, supports this hypothesis. For example, there are evidences that
adaptation to high altitude hypoxia among modern day Tibetans is a result of
introgression from Denisovans.
The populations of eastern
Indonesians, Papuans, Philippine "negritos," Siberians, South Asians,
and East Asians, are among the few existing groups that exhibit substantial
traces of genetic material from Denisovans. It is worth noting that the region
of Island Southeast Asia and Papua which consists of numerous densely inhabited
archipelagos, holds some of the earliest evidence of early archaic humans
outside Africa. Additionally, it is believed that archaic hominins like Homo
floresiensis coexisted with modern humans in this region.
Modern Australian aboriginials
and Papuans have the highest proportion of arround 5 to 6% of Denisovan
ancestry, while American and mainland Asian populations have a smaller
proportion about 0.2%.
The genetic diversity observed
within the Denisovan lineage suggests their deep divergence and separation into
three distinct branches across different regions. Three branches, D-one, D-two,
and D-zero, have been identified. Branch D-two contributed to the introgression
signal in Oceania and, to a lesser extent, in Asia. Branch D-one appears to be
primarily confined to New Guinea and nearby islands, while branch D-zero is
found in East Asia and Siberia. This indicates that Denisovans were capable of
crossing significant geographical barriers, inhabiting a wide range of
environments. The subgroups D-one and D-two diverged from the Altai Denisovan
lineage approximately two hundred and eighty thousand and three hundred and
sixty thousand years ago, respectively.
The genetic mixing with D-one
Denisovans, restricted to New Guinea and nearby islands, may have occurred as
recently as the end of the Pleistocene epoch, making them among the last
surviving archaic hominins in the world.
There is an argument that
interbreeding between humans and Neanderthals occurred east of the Wallace’s
Line and another group argues that interbreeding
occurred before humans crossed the Wallace’s Line.
The distribution of
Denisovan DNA in modern populations presents suggests significant interbreeding
may have occurred after early humans crossed Wallace’s Line. Indigenous groups
in New Guinea, Australia, and nearby islands exhibit the highest levels of Denisovan
ancestry, reaching 3 to 4 percent. Interestingly, populations in mainland Asia,
despite Denisovans’ historical range, show lower levels of Denisovan DNA.
This disparity indicates that Denisovan-modern
human interactions were concentrated east of Wallace’s Line, likely shaped by
migration and ecological boundaries Mainland populations may have experienced
a "replacement" effect, where later waves of East Asian populations
diluted or replaced Denisovan DNA. Even in isolated groups like the Andaman
Islanders, who show no admixture with other populations, no Denisovan DNA has
been found.
Denisovans are
thought to have lived across both mainland and Island Southeast Asia, thriving
in diverse environments. During the Pleistocene, when sea levels dropped, vast
savannah regions on the Sunda shelf (now submerged) likely served as migration
routes and refuges for Denisovans during climate changes. This mobility may
have allowed them to interact with modern humans in Island Southeast Asia,
resulting in genetic mixing.
The only ancient human species
clearly known to have crossed Wallace’s Line before modern humans is Homo
floresiensis. This species remains mysterious, as scientists are unsure of
its exact evolutionary origins. Some studies suggest it evolved from an early
ancestor of Homo erectus, or perhaps from an even older and more
primitive human species. Evidence of stone tools on Flores, dated to over 1
million years ago, indicates that these hominins arrived on the island very
early.
However, it's difficult to link Homo
floresiensis to the Denisovans. The Denisovans had larger molars (teeth),
which don’t match the anatomy of Homo floresiensis. Moreover, genetic
studies suggest that Denisovans and modern humans shared a common ancestor
between one hundred seventy thousand and 1 million years ago. This timeline
makes it unlikely that Homo floresiensis, which had been isolated in the
Wallacea region for so long, was directly related to the Denisovans.
Given the evidence,
it’s likely that Homo floresiensis was a species uniquely adapted to its
small island habitat, evolving separately for over a million years. The
Denisovans, on the other hand, likely arrived later, around six hundred
thousand years ago, and spread across a wider area of Asia and Southeast
Asia. Some Denisovan fossils may yet be undiscovered, but possible candidates
could include the Homo luzonensis
from Philippines or other mysterious
human fossils found in places like Narmada and Dali, Jinniushan, Maba, and
Xujiayao (China). These sites suggest that Denisovans, or other ancient human
species, once had a much broader range than previously thought. Recently evidence
of human activity has in Kashmir valley, where around 300 to 400 thousand years
ago humans relied on mammoth carcasses further supports this claim.
The Denisovan genome reveals evidence of interbreeding with
another ancient population, but the identity of this group remains a mystery.
Scientists speculate that Denisovans might have mixed with older human species
like Homo erectus, Homo antecessor, or
possibly a late-surviving Homo heidelbergensis in Asia. This
raises intriguing questions about how many ancient human groups coexisted and
interacted.
The fossil evidence from East Asia is incomplete and
scattered, but it hints at a rich diversity of hominin species. Discoveries
like Homo floresiensis on Flores and the Homo luzonensis remains
from Callao Cave in the Luzon island of Philippines suggest that multiple human
groups once inhabited Wallacea. This raises the possibility that some early
hominins might have even reached New Guinea and Australia. If true, it would
mean ancient humans were far more adaptable and capable of long-distance
migration than previously thought.
Gene flow between Denisovans and modern humans occurred
primarily east of Wallace’s Line, likely because the circumstances of these
encounters were unique. Crossing Wallace’s Line required watercraft, which
suggests that the first modern human groups reaching Wallacea were small and
isolated. In such scenarios, interbreeding with the established Denisovan
populations may have been more likely, as smaller populations often rely on
intergroup mating to maintain genetic diversity. Additionally, genetic signals
from these early interactions would be preserved more easily in the descendants
of such small founding populations.
Interestingly, evidence suggests that this gene flow was
predominantly male-mediated, meaning Denisovan males may have interbred with
modern human females. This provides clues about the nature of these
interactions, though much remains speculative.
Rapid dispersal into the tropical regions of Wallacea likely
exposed early modern humans to unfamiliar pathogens. Hybridization with
Denisovans, who were already adapted to the local environment, may have
provided disease resistance genes that offered a survival advantage. This could
have made interbreeding beneficial and increased the likelihood of these
genetic traits being passed on and preserved.
As genomic research progresses, it is expected to shed more
light on these interactions, revealing how Neanderthal and Denisovan DNA
contributed to the genetic makeup of modern humans and whether some of the
exchanged DNA had functional significance, such as aiding in disease resistance
or environmental adaptation.