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Mandel Ngan / AFP - Getty Images file
A skull from an ancient specimen of Homo sapiens (foreground, right) is compared with a Neanderthal's skull at the Smithsonian's National Museum of Natural History in Washington. Researchers suggest that a gene linked to the immune system played a roundabout role in brain evolution.
Scientists say our genes contain the hints of an evolutionary tug of war that took place in the wombs of our ancestors, balancing the drive to bigger brains with the need for a strong immune system.
The push and pull of these genetic variants apparently became more pronounced after pre-humans branched off from the ancestors of chimpanzees, according to biologists Peter Parham of Stanford University and Ashley Moffett of the University of Cambridge.
Two years ago, Parham and other researchers suggested that interbreeding with now-extinct cousins such as Neanderthals and Denisovans may have given early humans a boost of immunity. Parham says the same kind of cross-species hanky-panky may have played a role in the genetic diversity that he and Moffett discuss in a paper published online by Nature Reviews Immunology.
"It quite nicely dovetails with all this other stuff," Parham told NBC News. "There is an inherent instability in the way the underlying mechanism works."
How natural killers work
The two biologists focus on how particular types of white blood cells, known as natural killer cells, work in the human immune system. In addition to fighting infections and tumors, natural killer cells help regulate the growth of the placenta during pregnancy. Humans are unique among primates in having two variants of the genes that control the receptors for natural killer cells.
"B haplotypes are favored during reproduction. A haplotypes are more specialized toward defending against infections," Parham explained. "These are subtle effects. On average, if you're an individual that has two A haplotypes and no B haplotype, you're going to have a slightly more robust immune system in terms of dealing with disease."
Having two B haplotypes, in contrast, would allow for a more robust placenta. That would provide the fetus in the womb with more of the nutrients needed to grow a bigger brain. "In the course of human evolution, you had the evolution of these B haplotypes, which really did enable the brain to get bigger. ... There are correlations between the size of the brain of the baby and these genetic factors," Parham said.
A detailed analysis of human genetic diversity suggests that the genes for the B haplotype emerged in the time frame lasting from about 7 million years ago to 1.7 million years ago. That would cover a period starting with the divergence of human and chimp ancestors, and ending with the human migration out of Africa.
The A-vs.-B breakdown is found in all present-day human populations, suggesting that both variants were important to have for different situations. Parham and Moffett speculate that the A variant was important when a population was facing a disease epidemic, while the B variant became important for brain-building once the epidemic passed.
The role of the birth canal
When our ancestors began walking upright, that introduced another push-pull effect for brain size. "It's difficult to document, but it's generally thought in the field of obstetrics that birthing is more difficult for humans than it is for other species," Parham said. The dimensions and layout of the human birth canal is one constraint: If a baby's skull were to get significantly bigger, it wouldn't fit through the canal.
Scientists in Germany have captured the first video of a childbirth using an MRI scanner. TODAY.com's Richard Lui reports.
Another constraint is pregnancy's effect on the mother's cardiovascular system. In some situations, a potentially fatal condition known as preeclampsia can occur.
"Part of the compromise is that the human population has tolerated a certain amount of death in childbirth, due to obstructed labor or preeclampsia. ... Both of these types of death in childbirth have been quite common in our species, as has been documented in so many 19th-century novels," Parham said.
The genetic record indicates that the human species passed through a series of "bottlenecks" in prehistoric times that reduced population diversity to perilously low levels. That's where interbreeding with Neanderthals could have played a part. "One way that modern humans replenished the genetic diversity lost in populations was through the selection of new variants ... another, and possibly more effective, mechanism was to acquire old variants by mating with archaic humans," Parham and Moffett write.
Today, modern medicine has leveled the evolutionary playing field. But in ancient times, all these genetic and physiological factors seem to have interacted to make our brains what they are today.
"Basically, we've got the nervous system and the brain putting pressure on the immune system and the reproductive system," Parham said.
More about human evolution:
- How sex with Neanderthals made us stronger
- Where did we get the energy for big brains?
- Brawn may have boosted the human brain
Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.
Leave a Comment:
We must clone Neanderthals in order to mate with them and dumb ourselves down. It's the only way to save humanity and the Earth from our own oversized brains.
To the Bat-lab, Robin!
I wonder about that. Every human on earth except subsahara african has neanderthal genes ... it would appear to make humans smarter.
Are you sure you're not just a racist? Put that genious neantherthal DNA to work and get back to me.
The cloning is entirely unneccessary, the dumbing down is already progressing nicely....
Both Neanderthal and that other skull probably came from populations with larger average brain size than modern populations. But there are so many people alive today with so much variation that some people today have brain size about as large as any ancient person ever had. As well as some people today with much smaller size. And I think brain size does correlate with body size or at least height. While intelligence might be hard to correlate to head/brain size.
A possibility is that the "handsome" skull was from a population already influenced by Neanderthal mixing in Europe or the Middle East. As far as is known,Neanderthal showed archaeological evidence of burial of the dead and ceremonial grave goods before "modern" looking man of contemporary to that time? Also evidence of support of certain individuals too old or arthritic to hunt for themselves? (care of elders)
the more modern looking ancient skull in the foreground has eye sockets that are more roofed over or vertically compressed than typical of most modern skulls?
yah, too big a head and you don't get born, kinda cuts down the evolutionary ladder that. so next step is better use of what we got room for.
It seems that the researchers in this study hinge on the interbreeding with Neans.. That aspect of our history is not only in dispute but for many have been disregarded as a factor in mans history:
http://www.sciencedaily.com/releases/2012/08/120813155521.htm
The article di state that this "tug of war" could have taken place without interbreeding.... I'll just lean toward the "Master plan" for man simply culminated in us being the dominant species that we are today.
Plan? There ain't no plan.
I agree Flame. I kind of came to the article based on the caption, which was completely misleading. I was interested in the immune system of today and wondered about the tug of war between that and intelligence. Turns out, the article is a continued argument about evolution. But hey, why put that out front? Just give us a misleading caption.
Having a super immunity system would be nice. But, I would prefer a big brain. It would be better if we had both.
They may find out that this has happened in many animals and not just man. Crocs. have fairly small brains but an excellent immune system so good that there are no known diseases that will kill them. I believe that the Neanderthals acutally had a larger brain than our ancestors. I am not aware that Europeans, who are suppose to have had relations with the Neanderthals, have a larger brain than people in Africa. Was it a larger brain or the wiring of the brain?
Civil War era USA people were on the average lighter and shorter than today-The larger size of people today is blamed on more and better food and less severe conditions for growing up-I wonder if the average brain size is up from the Civil War era as well-if so probably also is response from different growing up conditions and more/better food.
Makes me think of the stereotype smart and precocious, but sickly, child. Or the stereotype big and healthy, but dumb, jock? Any connection, you think?
Well I survived many childhood illnesses and developed an above average IQ, doesn't mean much. Healthy and wise but not wealthy but not depending on handouts like so many voters today.
Interesting. What I get from this article is that genetic engineering is quite old.
Bigger or smaller , it's much better than no brains .
I would like to have the immunity of a crocodile but the brains of Einstein.
My take: this is like 1% science and 99% speculation. No point in taking any of this article's drivel as anything but a gleam in some academic's eye that hopefully will result in someone deciding to give them money. It's ALWAYS about the MONEY. Nature is subtle but sublime and we are usually ignorant fools.
Nobody is going to know the reality until there are time machines; in other words, probably never.