Author: Zahoor Ahmad
Modern humans did not evolve in complete isolation. Thousands
of years ago, early humans migrated out of Africa and encountered other human
groups such as Neanderthals and Denisovans. Over time, these groups interbred.
As a result, small amounts of ancient DNA remain in many people today.
Scientists now believe that inherited DNA from these ancient
populations may influence several modern traits, including skin
characteristics, immunity, pregnancy, metabolism, mental health, and
susceptibility to certain illnesses.
This article explains how Neanderthal and Denisovan DNA
entered the modern human genome, what researchers have discovered, and why
these findings matter today.
Researchers believe that anatomically modern humans first
evolved in Africa more than 100,000 years ago. Around 60,000 years ago, groups
of humans began migrating from Africa into Europe and Asia.
During these migrations, modern humans encountered other
human species already living in these regions, including:
Genetic studies show that these groups occasionally interbred.
Because of this, many people today still carry a small percentage of ancient
DNA.
In general:
The discovery of Neanderthal and Denisovan DNA became
possible after scientists successfully sequenced ancient genomes.
One of the most important researchers in this field is Swante
Pääbo, whose work helped reveal that interbreeding occurred between ancient
humans and modern humans.
By comparing ancient DNA with the genomes of living people,
researchers found clear signs that ancient genes were passed from one group to
another.
These studies also showed that ancient DNA is not spread
evenly throughout the human genome. Some regions contain more Neanderthal DNA
than expected, while other regions contain very little or none at all.
This pattern suggests that natural selection removed some
ancient genetic variants that were harmful while preserving variants that
offered benefits.
For many years, scientists focused mostly on Neanderthal DNA
in Europe and Asia. More recent studies suggest that ancient interbreeding may
also have occurred within Africa.
Researchers studying sub-Saharan African populations
discovered unusual genetic segments sometimes called “ghost haplotypes.” These
DNA patterns are difficult to explain using only modern human ancestry.
Scientists believe these segments may have come from
interbreeding between modern humans and other ancient populations that lived in
Africa.
Unlike Neanderthals and Denisovans, these African archaic
groups have not yet been fully identified because there is very little fossil
DNA available.
Even so, the evidence suggests that human history is more
complex than previously believed. Modern humans likely exchanged genes with
several different ancient populations over thousands of years.
Not all inherited Neanderthal genes were harmful. In some
cases, these genes may have helped early humans adapt to new environments
outside Africa.
Neanderthals had lived in Europe and western Asia for
hundreds of thousands of years. They were already adapted to colder climates,
different diets, and unfamiliar diseases.
When modern humans arrived in these regions, inherited
Neanderthal genes may have provided advantages such as:
Because these traits may have improved survival, they
remained in the human genome and were passed down through later generations.
Modern research shows that ancient DNA may influence a
variety of health conditions.
Some inherited variants appear to be helpful, while others
may increase the risk of disease.
Several studies suggest that certain Neanderthal gene
variants are associated with mood disorders, including depression.
Researchers found that some Neanderthal DNA regions occur
near genes involved in brain function and emotional regulation. Although these
variants do not directly cause depression, they may slightly influence a
person’s risk when combined with environmental and lifestyle factors.
Scientists have also observed links between ancient DNA and
neurological traits such as stress response, behavior, and addiction.
Some Neanderthal genes appear to influence blood clotting and
inflammation.
Researchers identified inherited variants that may affect how
quickly blood clots form and how the body responds to injury or infection.
While these traits may once have been useful for survival, in modern
environments they may increase the risk of:
Ancient DNA also appears to affect skin tone, hair texture,
and the way the body responds to sunlight.
Researchers believe these traits became useful as early
humans moved into regions with lower sunlight levels.
Some Neanderthal variants have been linked to:
These inherited
traits may have helped humans adapt more quickly to different climates.
Scientists discovered that some ancient genes influence how
the body processes nutrients.
One example involves a gene related to thiamine, also known
as vitamin B1. Changes in this gene may have helped early humans adjust to new
foods and diets after leaving Africa.
Other inherited variants may affect:
Not all Neanderthal DNA is harmful. In fact, some inherited
variants may provide important benefits.
A large study involving more than 244,000 women in the United
Kingdom found that women carrying a specific Neanderthal-derived gene variant
were:
Researchers discovered that this genetic variant affects
progesterone receptors in the body. Increased sensitivity to progesterone may
help support pregnancy during its earliest stages.
This is one of the clearest examples of an ancient gene still
providing a measurable benefit in modern humans.
During the COVID-19 pandemic, researchers discovered that a
region of chromosome 3 inherited from Neanderthals may increase the risk of
severe COVID-19 infection in some individuals.
This finding does not mean that Neanderthal DNA alone causes
severe illness. Many other factors such as age, overall health, and medical
history also play a major role.
However, the discovery shows that ancient DNA can still
influence how modern humans respond to infectious diseases.
Scientists have noticed that certain parts of the human
genome contain very little Neanderthal or Denisovan DNA.
These regions may have lost ancient DNA because those
inherited variants were harmful.
For example, some areas of the X chromosome and chromosome 7
contain much less Neanderthal DNA than expected.
Researchers believe natural selection gradually removed these
variants over time because they may have affected fertility, reproduction, or
other important biological functions.
This explains why only certain ancient genes survived while
others disappeared.
Research into ancient DNA is still developing. Scientists
continue to study how these inherited variants affect human health and
behavior.
Current evidence suggests that ancient DNA may influence:
However, most human traits are influenced by many genes and
environmental factors together. Ancient DNA is only one small part of a much
larger picture.
Future studies may reveal additional ancient populations and
explain how interbreeding shaped the genetic diversity seen in humans today.
The discovery of Neanderthal and Denisovan DNA has
transformed our understanding of human evolution.
Rather than replacing ancient human groups completely, modern
humans interacted and interbred with them. Those ancient encounters left a lasting
genetic legacy that still affects people today.
Some inherited variants may increase the risk of certain
diseases, while others continue to provide benefits such as stronger immune
responses or improved reproductive health.
As scientists continue to study the human genome, ancient DNA
will remain an important key to understanding where we came from and how our
biology continues to evolve.
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