Don't like the way you look, or the way you feel? Been blaming your parents and grandparents? Well, maybe you've been blaming the wrong folks.
Every day, more and more people discover their true genetic ancestry, thanks to ever lower cost DNA test from companies such as 23andme. Oftentimes, there are unexpected surprises. My wife, who was born in Italy - and whose entire family appears to have lived in Italy for generations – discovered that her father's ancestors were Vikings!
All these genetic tests provide us new sources of blame for things we don't like about ourselves – someone other than our immediate family. Besides checking the DNA of curious people like my wife and others, scientists are doing the same with for those long deceased. It's been reported that the genomes of several Neanderthals and one Denisovan have been sequenced. The evidence shows that these various groups interbred at various points in history. The implications of that inter-breeding are now becoming apparent.
You've probably heard jokes about the Neanderthals – and Neanderthal behavior – for years and years. Evidence now suggests that anyone of European ancestry has anywhere from 1% to 4% of their DNA from the Neanderthals – the result of all that interbreeding.
Likewise, if you can trace your ancestry to Oceania – meaning one of the South Pacific Islands – or to various parts of Asia, there's a good chance you have DNA traceable to a group called the Denisovans. For example, Aborginal Australians can trace about 3% to 5% of their DNA to the Denisovans. The same is true for people from Melanesia and parts of China and Tibet.
Conversely, if you're from Africa and have neither European nor Asian ancestry, you'll likely have neither Neanderthal nor Denisovan DNA.
Neanderthals were a species very similar to Homo sapiens, our species. Some make the argument that the Neanderthals were just close cousins to Homo sapiens. Both species have a common ancestor who lived about 500,000 years ago. While Homo sapiens differed somewhat from Neanderthals, we were sufficiently similar that the two species could mate and have children, thus explaining why so many of us have some Neanderthal DNA.
Neanderthals apparently moved out of Africa first. Then about 50,000 to 80,000 years ago, Homo sapiens also moved out of Africa. Homo sapiens and Neanderthals apparently only began to interbreed after the two groups moved out of Africa, thus explaining why the typical African doesn't have Neanderthal DNA.
We know a lot less about the Denisovans than about the Neanderthals. That's largely due to the fact that only a few fragments of deceased Denisovans have ever been found – nothing more than a finger bone, a toe bone, and several teeth – from a cave in remote Siberia.
When two populations, such as the Homo sapiens and the Neanderthals, begin to interbreed, it's called "adaptive introgression". So what are these DNA studies revealing about adaptive introgression? A number of interesting things.
First, it may explain the real reason why you look as you do. One study identified some twenty different physical traits in modern humans that are traceable to the Neanderthals. If you happen to have rosy cheeks, little or no protruding chin, or a broad projecting nose, don't blame Aunt Martha or your father, thank one of your Neanderthal ancestors instead.
But of potentially greater interest are several recent DNA studies suggesting that Neanderthals not only provided some of us with rosy cheeks, they also gave many of us allergies, as well as an increased risk for depression. Another recent study looked at the health records of Americans and concluded that Neanderthal admixed DNA may affect the risk of depression; explain skin lesions resulting from sun exposure; explain hypercoagulation of blood; and even explain something about tobacco use.
Having trouble quitting smoking? I guess you can blame it on that nameless ne'er do well Neanderthal ancestor of yours!
But the Neanderthals didn't just give some of us a genetic variation of the "lump of coal" for Christmas. In fact, two other new studies identified three archaic genes from the Neanderthals that boost immune response, a benefit.
Other studies show the same positive benefit resulting from interbreeding between Homo sapiens and Denisovans. One positive benefit of that is evidence that the Denisovans provided certain groups in Tibet the ability to survive and thrive at high altitudes. Anyone who has ever travelled to Pikes Peak in Colorado (elevation 14,110 feet above sea level) or the base camp in Nepal from which teams begin the ascent of Mt. Everest (elevation 18,500 feet above sea level) knows how hard it is for humans to breathe at such altitudes. However, the Denisovans provided at least some modern day Tibetans the right genes for this.
Darwin's theory of evolution by natural selection posits that "bad genes" will eventually disappear. If that's the case, then why would bad genes that lead to depression and allergies, for example, persist? There are two likely answers.
One reason "bad genes" might persist for such a long time is because they were likely beneficial in prehistoric times. We obviously live in a very different world than did our Neanderthal, Denisovan, or Homo sapiens forebears. The conditions that made those "bad genes" beneficial have disappeared. The genes may be bad in a modern environment, but not bad enough that they would prevent the person with the bad genes from having offspring. After all, you may have nasty allergies, and life may be a bitch in allergy season, but how many people with allergies do you know who died before they could pass on their genes?
The other reason may be related to modern day lifestyles and environments. Take the case of allergies. There's some evidence that allergies are problematic for modern humans because of the conditions in which we live. Another is that children are raised in overly sanitary environments, so they do not develop immunity from a range of minor bugs while they're young and their immune response somehow remains immature.
At the same time, while the Neanderthals provided us some bad things, another new study
suggests that evolution by natural selection did in fact work: large numbers of Neanderthal gene variants that would be deleterious to humans were purged. But researchers made another interesting discovery: that "purging" wasn't a foregone conclusion. Instead, the purging of many of these Neanderthal genes occurred only because the Homo sapiens population was so much larger than the Neanderthal population. If, instead, the Homo sapiens and Neanderthal populations had been more evenly balanced, not only would more us have rosy cheeks, we'd also have more Neanderthal genes, including ones that are mildly deleterious to modern day humans. According to Ivan Juric of the University of California Davis, "Selection is more efficient at removing deleterious variants in large populations." The study further noted, "Weakly deleterious variants that could persist in Neanderthals could not persist in (early modern) humans. We think that this simple explanation can account for the pattern of Neanderthal ancestry that we see today along the genome of modern humans."
What that implies is a potentially very different outcome if the populations of Homo sapiens and Neanderthals had been more evenly balanced. We likely would have a lot more Neanderthal DNA than we do. Alternatively, we might have a lot less Homo sapiens DNA than we do. Taking that a step further, maybe we might actually be mainly Neanderthal, not Homo sapiens?
Of course, passing good and bad genes isn't a "one way street". I've noted the evidence that Neanderthals have bequeathed upon many of us some not so nice DNA. According to another recent study, Homo sapiens returned the favor by passing along to Neanderthals the bacterium that causes such truly pleasant things as stomach ulcers, tapeworms, and tuberculosis. The Neaderthals apparently hadn't developed resistance to such bugs, so exposure was catastrophic, much as the introduction of various bugs by the Spanish explorers to the Americas in the late 15th and early16th centuries was catastrophic to native populations.
Our individual genomes carry surprises of both the good and the bad variety. Not only that, they shed light on the rich journey of Homo sapiens and related species from the time of the emergence of Homo in Africa. So be ready to be surprised even more.
