Infectious disease may have shaped human origins, study says

Jun 04, 2012
Escherichia coli bacteria, like these in a false-color scanning electron micrograph by Thomas Deerinck at UC San Diego’s National Center for Microscopy and Imaging Research, cause a variety of often life-threatening conditions, particularly among the young. Varki and colleagues suggest a genetic change 100,000 or so years ago conferred improved protection from these microbes, and likely altered human evolutionary development. Credit: Thomas Deerinck at UC San Diego’s National Center for Microscopy and Imaging Research

An international team of researchers, led by scientists at the University of California, San Diego School of Medicine, suggest 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 coli K1 and Group B Streptococci, the leading causes of sepsis and meningitis in human fetuses, newborns and infants.

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.

Add another possible factor: infectious disease.

In a paper published in the June 4, 2012 online Early Edition of The , an international team of researchers, led by scientists at the University of California, San Diego School of Medicine, suggest that inactivation of two specific genes related to the immune system may have conferred selected ancestors of with improved protection from some pathogenic , such as Escherichia coli K1 and Group B Streptococci, the leading causes of sepsis and meningitis in human fetuses, newborns and infants.

"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 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 . can then depend upon either resisting the pathogen or on eliminating the 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 .

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."

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Caliban
5 / 5 (1) Jun 04, 2012
And those same forces are still at work today.

It's not too difficult to imagine a pathogen decimating the human population in the modern era. If, for instance, HIV had been only a couple of ticks more rapid in progression, we would be looking at the results of just such a force, even as we speak.
Probably, given this context, only a matter of time.

kaasinees
2 / 5 (4) Jun 04, 2012
There is a cure for HIV, bone marrow transplant and a cocktail of drugs can do the trick.

And there is still the rule that 1% of the species is immune to the disease. Also not everyone is a whore.
ormondotvos
not rated yet Jun 04, 2012
A rule? Got a link?

Where does the money come from for the cocktail and transplant?

Smallpox?
dtxx
not rated yet Jun 04, 2012
Smallpox? Go check out Chagas. This is a hasty generalization to be sure, but it's basically AIDS spread by insect bites (it's actually an incurable parasite that causes the heart and intestines to burst). Chagas is believed to have killed Charles Darwin, but apparently it's spreading rapidly now.
kevinrtrs
1 / 5 (13) Jun 05, 2012
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.

How about a global flood? Would that also count as a cause?

Now besides the bottleneck mystery, here's another one:
Just how would the human immune system develop via evolutionary processes?
Consider some of the aspects involved, all of which need to be present simultaneously or it doesn't work.
A means of detection of invasion. A means of signalling a far-removed source of clean-up agents, i.e. killer cells etc. A means of transporting those cleaners to the site of infection. A means of leaving the transport mechanism and entering the infected site.

Now just on that last point - consider the incredible complexity involved in neutrophil extravasation with the almost miraculous transformations the cell has to undergo. "It Evolved?" - I don't think so. Far too complex.
alfie_null
4.8 / 5 (6) Jun 05, 2012
Now just on that last point - consider the incredible complexity involved in neutrophil extravasation with the almost miraculous transformations the cell has to undergo. "It Evolved?" - I don't think so. Far too complex.

Only thing this (and pretty much the rest of the post) communicates to me is that some people have limited cognition; less ability to grasp complexity. Perhaps intentional ignorance.
To paraphrase: "I don't understand it, therefore it is impossible".
Ojorf
4.6 / 5 (5) Jun 05, 2012
How about a global flood? Would that also count as a cause?

It would have if it could have happened, but since it didn't it couldn't, so no.
Aira_Moonshade
5 / 5 (1) Jun 05, 2012
@Kasinees " Also not everyone is a whore." This must be the most narrow minded opinion I have heard all day. Tell that to the woman and children getting raped in South Africa, whoring were they??
http://digitaljou...e/264771
davhaywood
not rated yet Jun 10, 2012
Immune systems didn't miraculously arise in humans. They were developed by predecessors of humans for billions of years before the human species even existed. Again, Lenski, Lenski, Lenski. His experiments demonstrate how seemingly "useless" mutations can arise and survive without apparent function, only to be incorporated into a later system with other mutations. I truly wish Creationists create their own state, secede from the international community and demonstrate how they intend to advance scientifically with their backward approach.