Two hundred thousand years ago or thereabouts, an African lion killed someone. Along with a meal, the big cat got a wicked stomachache. Today a record of that unfortunate death still survives, in the bacteria that make big cats sick.
The trail of this strange story starts in the 1980s, when scientists discovered that ulcers are caused by bacteria known as known as Helicobacter pylori. H. pylori is found in people around the world, and scientists learned how to recognize the different strains they carried. Based on the patterns of the strains, a team of scientists concluded in 2003 that Helicobacter pylori must have been present early in the history of our species, and was spread across the world during the migration of humans. (I wrote a long post on H. pylori and human evolution when the scientists who discovered its link to ulcers got the Nobel Prize.)
But there were skeptics.
The skeptics proposed that Helicobacter pylori spread into humans in just the past few thousand years. They searched for a closely related species of Helicobacter that might have been its ancestor and found one: Helicobacter acinonychis. Preliminary studies suggested that it was far more like our own Helicobacter pylori than other species of Helicobacter. Helicobacter acinonychis also causes severe gastritis in its hosts--lions, tigers, and cheetahs. In laboratory experiments, scientists found that it could also infect mice.
Some of the scientists who had initially proposed the ancient origin of Helicobacter pylori have tested this alternative, and they've published their results in a paper in the journal PLOS Genetics. Their surprising conclusion is that the big cats actually got their ulcer bugs from us.
The scientists got to this conclusion by sequencing the genome of H. acinonychis and then comparing it to H. pylori strains. By comparing the mutations in each strain, the scientists found that the bacteria all descend from a common ancestor that lived about 200,000 years ago. But which way did the parasite jump? The scientists argue that it went from humans to cats. The cat bacteria carry 92 fragments of various genes that can be found in good working order in the human strains. It would be hard to imagine how the bacteria could have moved from cats to humans and then repaired all 92 genes. The bacteria must have shifted from humans to cats, and then genes that were no longer essential in the new environment began to mutate. Many of the genes that Helicobacter acinonychis has lost made proteins that appear on the surface of Helicobacter pylori. Losing those proteins may have helped the bacteria escape the immune systems of cats. The bacteria also have some genes not found in Helicobacter pylori, and these show signs of having been delivered to the bacteria by viruses. Exactly what they do for the microbes isn't clear yet.
What makes this hypothesis particularly compelling is that it's the same sort of process that scientists have documented in many other species of bacteria. It just so happens that scientists have recently documented it in the evolution of bacteria in yogurt 5,000 years ago, about which I blogged last week.
It's pretty easy to guess how a lion or cheetah could have picked up our bacteria: by having one of us for breakfast. Once the bacteria were transferred to a big cat, they then spread to other species. It's no big surprise that big cats ate some of our ancestors. (Check out the recent book Man the Hunted for the evidence.) What is surprising is how clearly the unfortunate fate of some early Homo sapiens has been preserved in living bacteria. And it's also a striking demonstration that it's not just humans that can pick up diseases from animals (such as influenza and HIV). The parasite highway runs both ways.
Comments
Finally, an example of humans striking back. All we ever hear about is some nasty bug jumping species boundaries to humans. I'm glad we got off our collective butts and struck back especially with a preemptive strike.
Posted by: Bruce Thompson | June 18, 2006 1:22 PM
Would we be better off as yogurt-eating vegetarians?
Posted by: David B. Benson | June 18, 2006 4:13 PM
I know I would be.
Posted by: Tim | June 19, 2006 3:46 PM
Makes you wonder what Helicobacter-infested creature our ancestors ate to get this bug into our lineage in the first place.
Posted by: Greg | June 19, 2006 10:10 PM
David B. Benson (June 18, 2006 04:13 PM) asks:
Would we be better off as yogurt-eating vegetarians?
Not clear, but we (or our ancestors) would probably have been better off if big cats were.
Posted by: marcel | June 20, 2006 2:24 PM
"Now you kittens listen to me: Don't you eat those hairless apes--they're filthy!"
Posted by: tgibbs | June 25, 2006 11:59 AM
This paper tells a similar story to Hoberg et al's "Out of Africa - Origins of the Taenia Tapeworms in Humans", but, it seems, from a quick read, more rigourous, better informed and a lot less speculative.
Hoberg et al 'proved' that humans must have caught tapeworms long before domestication (therefore we gave them to pigs and cows) and as a result of eating bovids, shared as prey with lions, cheetahs, hyaenas and hunting dogs.
They presented figures purporting to show that speciation between two human tapeworm species closely related to felid and hyaenid tapeworms diverged 'between 780kya and 1.71Mya'.
These were figures taken up by many commentators, including, I'm sorry to say, Carl Zimmer.
They 'proved' that humans ate big game regularly, at least as long ago as a million years, and depending on your bias, perhaps as long as 2.5My.
Those dates have irrevocably entered the 'Human Story' as the paper is now widely cited by palaeoanthropologists.
Hidden in the small print of the paper were the real estimates of bracketed ages (to 95% confidence level) ranging between 170,000 years ago and infinity (ie before Creation).
I've written this up at:
http://www.coconutstudio.com/Tapeworms.htm
as part of a series of 'papers' querying the general assumption that very early humans ate a lot of big game meat, and thus we became what we are.
The writers of this paper are far more cautious, and note that rates of gene evolution in bacteria can vary widely.
Hoberg et al also noted similar rate changes between larger 'animals', but then based their estimates of the timing of speciation in tapeworms on comparisons between their 'molecular clock' ticks and those of hammerhead sharks (diverged when Panama closed), humans/chimps, snapping shrimps, and mangrove crabs. Perhaps if they'd mentioned the common names of those in their paper they might not have got such unquestioned acceptance of their findings.
Finally, ignoring the fact that lions, leopards, and hyaenas, the primary hosts of tapeworms closely related to human ones, ranged throughout Africa and Eurasia (even North America) during the Pleistocene, they plumped for origins of tapeworms in Africa alone via African bovids (antelopes) as secondary hosts.
They didn't actually give any direct evidence that the antelopes they named were the exclusive secondary hosts, nor that they were commonly infected in the wild. In fact their list of secondary hosts appears to come from a couple of palaeoanthropogical papers quite unrelated to tapeworms.
African antelopes nowadays are not infected with specifically human tapeworms, which might be another slight objection to their findings.
Yes, we were definitely more often eaten by, rather than catching and eating, big game.
Richard Parker
Posted by: Richard Parker | July 5, 2006 2:00 PM
Just a quick question.
How did you like 'Man the Hunted'? I read some decent reviews and one scathing one on amazon.
I'm curious to your thoughts.
Posted by: Chance | July 6, 2006 12:01 AM