Infectious Bacteria Altered Human Evolution 100,000 Years Ago

 

                   

 

Roughly 100,000 years ago, human evolution reached a mysterious bottleneck: Our ancestors had been reduced to perhaps five to ten thousand individuals living in Africa. In time, "behaviorally modern" humans would emerge from this population, expanding dramatically in both number and range, and replacing all other co-existing evolutionary cousins, such as the Neanderthals.

The cause of the bottleneck remains unsolved, with proposed answers ranging from gene mutations to cultural developments like language to climate-altering events, among them a massive volcanic eruption.

Now it appears that inactivation of two specific genes related to the immune system may have conferred selected ancestors of modern humans with improved protection from some pathogenic bacterial strains, such as Escherichia coliK1 and Group B Streptococci, the leading causes of sepsis and meningitis in human fetuses, newborns and infants according to an international team of researchers, led by scientists at the University of California, San Diego School of Medicine.

"In a small, restricted population, a single mutation can have a big effect, a rare allele can get to high frequency," said senior author Ajit Varki, MD, professor of medicine and cellular and molecular medicine and co-director of the Center for Academic Research and Training in Anthropogeny at UC San Diego. "We've found two genes that are non-functional in humans, but not in related primates, which could have been targets for bacterial pathogens particularly lethal to newborns and infants. Killing the very young can have a major impact upon reproductive fitness. Species survival can then depend upon either resisting the pathogen or on eliminating the target proteins it uses to gain the upper hand."

In this case, Varki, who is also director of the UC San Diego Glycobiology Research and Training Center, and colleagues in the United States, Japan and Italy, propose that the latter occurred. Specifically, they point to inactivation of two sialic acid-recognized signaling receptors (siglecs) that modulate immune responses and are part of a larger family of genes believed to have been very active in human evolution.

Working with Victor Nizet, MD, professor of pediatrics and pharmacy, Varki's group had previously shown that some pathogens can exploit siglecs to alter the host immune responses in favor of the microbe. In the latest study, the scientists found that the gene for Siglec-13 was no longer part of the modern human genome, though it remains intact and functional in chimpanzees, our closest evolutionary cousins. The other siglec gene – for Siglec-17 – was still expressed in humans, but it had been slightly tweaked to make a short, inactive protein of no use to invasive pathogens.

"Genome sequencing can provide powerful insights into how organisms evolve, including humans," said co-author Eric D. Green, MD, PhD, director of the National Human Genome Research Institute at the National Institutes of Health.

In a novel experiment, the scientists "resurrected" these "molecular fossils" and found that the proteins were recognized by current pathogenic strains of E. coli and Group B Streptococci. "The modern bugs can still bind and could potentially have altered immune reactions," Varki said.

Though it is impossible to discern exactly what happened during evolution, the investigators studied molecular signatures surrounding these genes to hypothesize that predecessors of modern humans grappled with a massive pathogenic menace between 100,000 and 200,000 years ago.

This presumed "selective sweep" would have devastated their numbers. Only individuals with certain gene mutations survived – the tiny, emergent population of anatomically modern humans that would result in everyone alive today possessing a non-functional Siglec-17 gene and a missing Siglec-13 gene.

Varki said it's probable that humanity's evolutionary bottleneck was the complex result of multiple, interacting factors. "Speciation (the process of evolving new species from existing ones) is driven by many things," he said. "We think infectious agents are one of them.

Journal reference: Proceedings of the National Academy of Sciences 

The Daily Galaxy via University of California - San Diego 

Comments

[Garibaldi]

Is this an excuse for another pandemic? Erasing the entire useless population? AIDS didn't work so good but think about Ebola! The best excuse will be the economic downturn, right? God may hunt you forever because there is a GOD!

[Travis T.]

Sir... Your comment is both unnecessarily accusational, and also very ignorant. First of all, most concepts of a "god" figure are benevolent. So they would not waste their times with such evil actions as hunting someone down forever. Don't you think that perhaps, your "god" if he existed, would have slightly more pressing matters to attend to than wasting eternity on hunting someone simply because they've done a little logical thinking and science, enough to understand that we're here because of biochemistry, and evolution? Are you actually ignorant enough to accuse scientists of creating pandemics and using them to mass murder innocent people? It seems to me that your religious beleif is having a negative effect on you sir. It's clouding your perception of the world, and your sense of logic. I propose for your own life quality's sake that you begin to question things, and learn about your environment. Open, nay... Unlock your mind to the vast quantities of knowledge awaiting your observation.

[ophu]

@Garibaldi: Prejudice and stupidity kill a lot of people. Why don't you pray for a cure?

[Peter Cope]

Thanks, Garibaldi. I'd rather put my trust in intelligent free thinkers who would be more likely to produce a cure. I can't see any benefit in praying - which in effect is begging - to some imaginary cloud person.

[Noochy]

@ Gribs - You clearly have no idea what pandemic even means lol. Too bad the bible doesn't have a dictionary section eh. Seriously though ... why do you even go on blogs regarding evolution?

[Saint]

I would do both. Put my faith in Human Intellect and put my faith in the creator

[Formless]

Assumptions, all around. I agree with Saint.

@Noochy-This blog refers to science, not just your dogma of evolution. Many others have as much interest here as evolution, and Darwin missed the mark by the way.

<>And Russia is not bigger than Africa. Do your research<>

[Ferdinand]

I´m not too religious...and I don´t that we have to pray for a cure...the only thing we have to do is to think..our actions needs just a little of common sense...on the other hand, it´s very interesting that capitalism and science and nowadays the religion, specially catholic, would act like and selective force...

[LAZARLIN]

THIS FITS IN WITH ADAM & EVE THE FIRST BLESSED (AND MOST LIKELY PURE GENETICALLY SPEAKING)& THEIR EXPULSION FROM EDEN...
Forced out into a wilderness of sick diseased and impure individuals,and for their offspring to mix the genetic code widely though their numerous children.
And also "with individual choice".... not. as the case maybe...

[LAZARLIN]

In essence ..the actions of Eve left impuities in gods plan for man otherwise we may have risen to greater height much sooner,unfortunately now has come the time of wheat & chaff sifting,on a physical as well as metephysical level...

[Ruth Mc]

Poor old Man eh? Ed: I think you might've missed a bit of chaff...

[Beelzebub]

Thou hast revealed thyself unto me, Garibaldi. I shall follow and you shall fall into perdition, for my deceptions shall overtake you. Place not your faith in the false God of your parents for they were fools, as are you. The days of your time draw nigh and darkness comes. I know you now and despair shall be your fate. Gibbering fool, I come for you...