Sex With Neanderthals: The Ultimate Immunity Boost

Labor Day has passed and the temperature in Chicago has dropped: it's starting to feel like fall. I'm beyond excited about boots, Halloween costumes, changing leaves, and pumpkin beer. I am not, however, excited about the inevitable case of The Sniffles I will acquire this season.

Fortunately, my healthy immune system will protect me from pesky colds. Good immunological health -- like any trait -- is dependent upon both nature and nurture. That is, many factors must interact to produce a given trait. A few of these factors, such as proper nutrition and adequate sleep, are tried-and-true immune system boosters. A diet that consists solely of frozen pizza and Cheez-Its will not help you fight disease. Nor will the sleep schedule of the typical medical student. (Sigh.)

Another key contributor to a good immune system is good genes.

Lately, researchers have been asking, Where did these "good genes" come from? The answer: sex with Neanderthals. Well, to be more accurate: ancient interbreeding with Neanderthals and Denisovans gave modern Homo sapiens the ability to fight certain diseases. The journal article, published in Science, is entitled "The Shaping of Modern Human Immune Systems by Multiregional Admixture with Archaic Humans."

This new research is very exciting to me. It's a great link between two fields I've been involved in during the past few years: immunology and anthropology. This past summer, I did research in an immunology lab. And as an undergraduate, I was an anthropology major. Three summers ago I catalogued human skeletal remains at Chicago's Field Museum of Natural History. During my breaks, I would scamper over to the evolution exhibit to spend time in the hominid evolution room. (I'm not a nerd, I swear.)

To understand the conclusions made in the Science paper mentioned above, you'll need crash courses in hominid evolution and immunology. Allow me to be your professor.

Paleoanthropology 101

1. Contrary to popular portrayals, Neanderthals were highly intelligent. They had language, tools, and complex societies.
2. Homo sapiens are not descendents of Neanderthals. Rather, the two species share a common ancestor and cohabited parts of the planet at certain points in history. Think of it this way: you and your cousin are related because you share common ancestors. You have the same grandparents, great-grandparents, etc. Importantly, your cousin is not your ancestor. (Unless you have a really weird family.) Homo sapiens and Neanderthals are cousins.
3. Denisovans and Neanderthals also share a common ancestor. Using the same analogy, Denisovans and Homo sapiens are cousins. Denisovans and Neanderthals, however, are more closely related to each other. They're like siblings. Cousins have common grandparents, but siblings have common parents.
4. Ok, this is where the cousin/sibling analogy has to end, because Homo sapiens, Denisovans, and Neanderthals all interbred with each other. If you are non-African, 1-6% of your genome is Denisovan/Neanderthal.
5. There are several theories regarding archaic human migratory patterns. One such theory postulates that some Homo sapiens migrated out of Africa 67,000 years ago. Roughly 10,000 years ago, some of these early humans migrated back to Africa.

Immunology 101

1. A key player in this research is HLA Class I. HLAs, or human leukocyte antigens, are molecules involved in the immune response. HLA Class I is particularly important in fighting viruses, like the one that causes the common cold. If you get sneezed on while riding the El and manage to avoid a case of The Sniffles, it's because HLA Class I molecules destroyed the bugs for you. That said, getting sneezed on is ill-advised.
2. HLA Class I comes in many forms and is inherited from our parents.
3. You and your partner have differences and similarities in your HLA molecules. The more similar you two are, the more likely your partner is to cheat on you.
4. The genes responsible for producing HLAs are called major histocompatibility complexes, or MHCs. A well-known Swiss study claims that women prefer the odor of men whose MHC genes are different from their own. Theoretically, MHC diversity should result in offspring with stronger immune systems.

Putting it all together...

Based on DNA analysis, Neanderthals and Denisovans had very similar HLA genes. Remarkably, present-day East Asians owe 70-95% of their HLA genes to interbreeding with Denisovans. Present-day Europeans owe roughly half of their HLA genes to interbreeding with Neanderthals. Present-day Africans owe roughly 6% of their HLA to Neanderthals/Denisovans, probably via later generations of hominids who travelled back to Africa. (Sidenote: it is likely that present-day Africans owe some of their immune system to interbreeding with non-Neanderthal/Denisovan early hominid species.)

The HLA genes Homo sapiens acquired via interbreeding likely helped them survive in a new climate, ripe with viruses their bodies had never encountered before. Migrating out of Africa into a colder climate would have been an immunological shock to the first Homo sapiens that populated Eurasia. After several generations of mating with the locals, however, mixed-species great-great-great-great-grandchildren were better equipped to fight indigenous viruses.

The takeaway point is this: early hominids interbred, leading to an improved Homo sapien immune system. So, next time you successfully beat the common cold, be thankful that your ancestors got freaky with members of other species.