She's our closest relative in evolutionary terms.

And this is a modern human male, or Homo sapiens, who arose in the same historical period as the Neanderthals.

For certain physical differences and other reasons, Neanderthals and Homo sapiens are classified as separate species.

And according to biologists, different species do not usually produce fertile offspring.

But by studying our human ancestors, scientists have found that the definition of species is not so clear-cut.

Although they were different, Neanderthals and Homo sapiens not only lived together, but also had sexual intercourse, and often.

And they did produce offspring, us.

But how did sex with Neanderthals change us?

We often think of human evolution as happening in a straight line.

But the reality is much more chaotic and fascinating.

Homo heidelbergensis, a species derived from Homo erectus, appeared in Africa approximately one million years ago.

After spreading across Africa, it went to Europe and Asia.

The heidelbergensis that remained in Africa gave rise to us, Homo sapiens.

Those that went to Europe evolved into Homo neanderthalensis, and those that went to Asia, into Homo denisova.

But the appearance of each new species did not lead to the death of the others.

Around 120,000 years ago, at least six species of the genus Homo coexisted.

And we're only talking about the species that we know of.

These groups of hominids migrated, joined together, and separated.

Some of these encounters were probably hostile.

Others clearly gave rise to families of different species.

So today, we know that everyone with Eurasian ancestry shares between one and three percent of their genes with the Neanderthals.

But discovering it was not an easy process.

In 2010, a group of scientists from the Max Planck Institute in Germany sequenced the genome of a Neanderthal fossil for the first time and compared it with ours.

That no Neanderthal fossils were ever found in Africa indicates that they interbred with Homo sapiens in Europe and Asia.

The most accepted theory is that Homo sapiens would have mixed with Neanderthals between 40 and 60,000 years ago, when they advanced from Africa to the north.

This hypothesis was reinforced in 2015 when the genome of the oldest sapiens found in Europe was sequenced and it was found to have between 6 and 9 percent Neanderthal DNA.

That fossil was between 37 and 42,000 years old.

But Neanderthal DNA didn't stay in Europe and Asia.

Contrary to what was believed, African populations also have some Neanderthal DNA.

This likely happened when Homo sapiens descended from interbreeding with Neanderthals, migrated back to Africa around 20,000 years ago.

And those genes we inherited from Neanderthals contributed to our evolution.

These genes influence how sensitive our skin is to the sun and how our hair grows.

Features that would have been essential when adapting to new environments.

And may have helped Homo sapiens when they migrated to Europe and Asia.

Another positive Neanderthal variant protects us from bleeding during pregnancy and prevents spontaneous abortions.

Other variants play a role in whether we are more active during the day or at night, in psychological traits such as depression, and in our propensity to nicotine addiction.

But there are genes that may have been useful at the time and can harm us today.

One example is a variant that contributes to faster blood clotting.

It may have helped wounds heal faster, but now the persistence of this gene increases the risk of blood clots and strokes.

Another study suggests that there is a relationship between our Neanderthal genes and COVID-19.

People with this variant may be two to four times more at risk of becoming seriously ill if they are infected with coronavirus.

Beyond genes, very little is known about what sex was like between these two species, even whether it was consensual or not.

But there are studies that are beginning to clarify these practical issues.

In 2017, an anthropologist from Pennsylvania State University identified traces of a type of bacteria that is present in our mouths in the tooth of a Neanderthal fossil.

She believes that this bacterium was transferred between Neanderthals and sapiens perhaps because they kissed.

There are even scientists who think that these two species may have exchanged sexually transmitted diseases, such as the human papillomavirus and herpes.

This same exchange may have been one of the factors that contributed to the Neanderthals disappearing, and not us.

Discoveries about interbreeding between species have given more strength to the so-called assimilation theory.

Since the population of Neanderthals was much smaller, they were absorbed into the Homo sapiens community over time.

According to this theory, as families of sapiens and Neanderthals merged, the distinct Neanderthal species eventually disappeared.

If you think about it, it's possible to say the Neanderthals didn't become completely extinct.

They are still alive, in a way, within us.

NASA Jet Propulsion Laboratory, California Institute of Technology