16 February 2007
Is Evolutionary Development Like Flat-Pack Furniture?
By Rusty Rockets
We may regard our Saturday morning trips to Swedish furniture shops as something quite trivial and mundane in the scheme of things, but piecing together your new flat-pack modular bookcase or chair may reflect something much deeper about our evolutionary development. Recent studies looking into the phenomenon known as horizontal gene transfer (HGT) - where genes from one species can be transferred to another - show that organisms can acquire add-on genes from sources other than their parents or ancestors. Pooh-poohed 50 years ago, HGT's modular mode of gene transfer may explain how evolution rocketed at certain points, and even why there is such a cognitive divide between humans and chimps.
Depending upon what you read, our chimp cousins share a percentage of our DNA that falls roughly somewhere in the high 90s, but if we are so alike, how did we end up living in skyscrapers, driving cars and venturing into space, while chimps remain in trees picking fleas off of one another? But before we attempt to answer that question, let's back up a bit, and give our cousins credit for showing at least some technological aptitude.
According to recent observations made by University of Calgary archaeologist Dr. Julio Mercader, chimp tool use can be dated back 4,300 years (an estimate based on some stone "hammers" he uncovered in the Taï rainforest of Africa's Ivory Coast). "It's not clear whether we hominins invented this kind of stone technology, or whether both humans and the great apes inherited it from a common forebear," says Mercader. "There weren't any farmers living in this region 4,300 years ago, so it is unlikely that chimpanzees picked it up by imitating villagers, like some scientists used to claim."
Too unwieldy for human use, the hammers, or rocks, are about the size of a cantaloupe, and bear unmistakable patterns of wear and starch residues consistent with being used to smash open the nuts known to be a staple food of chimps. Mercader explains that it takes a fair amount of skill to use such a tool, and it can take several years for a chimp to learn how to exert the thousand kilograms of force need to crack the shell without pulverizing the fruit within.
Apparently, Mercader's work on the material culture of great apes has created quite a hubbub around university water coolers, which has led to a whole new sub-discipline of chimpanzee archaeology opening up. "We used to think that culture and, above anything else, technology was the exclusive domain of humans, but this is not the case," says Mercader. But if chimps had the wits to master certain technologies, and devise complex social structures, why did humans, cognitively speaking, leave them for dead after the human-chimp split? What crucial evolutionary factor left poor old cousin Trog out in the wilderness?
What researchers do know is that for some reason the human brain developed differently to the chimp brain over the past 5 - 7 million years. "What sets us apart from chimps are our brains: homo sapiens means 'the knowing man'," says Professor Daniel Geschwind of the David Geffen School of Medicine. "During evolution, changes in some genes altered how the human brain functions."
Traditionally, such changes would have been solely put down to the slow, gradual change caused by environmental pressures and natural selection. Cognitive development hypotheses that have stemmed from this school of thought have been many and varied - including a change to nutrient rich seafood diets, climate change, and even psychoactive alkaloids from the root of Africa's Tabernanthe iboga - but now recent research into horizontal gene transfer (HGT) presents us with yet another fascinating possibility.
As it stands, evolutionary theory is a strong foundation on which to hang our knowledge of the natural world, but certain aspects of the theory have been bugging scientists for years. One of the most vexing questions is how the speed and complexity of evolution appears to be increasing as time goes by. What researchers find so baffling is how it took 2.5 billion years for multi-cellular life to evolve, and only a billion or so years for every complex organism that we now know of to populate the Earth. It's this situation that has led to the recent acceptance of HGT as a viable explanation, as it only requires that DNA be introduced from one species to another, and not via the slower process of inherited genetic information; although the latter still plays a major role. This process was first thought only to occur between bacteria, but it is now accepted in some quarters that HGT also occurs among larger animals and plant life. In fact, research has shown that plant parasites, bacteria and even viruses are the transporters of genetic information between species.
In 2004, after discovering that plant genes can move from plant to parasite and back again, biologist Jeff Mower remarked in Nature that: "Plant parasitism has emerged as the first solid mechanism of horizontal transfer in plants." Likewise, animals are also the benefactors of DNA carrying microbes, as recent research from Rice University shows. Rice's Michael Deem says that bacteria and viruses exchange transposable chunks of DNA between animal species all the time. And how do they know this? "We have developed the first exact solution of a mathematical model of evolution that accounts for this cross-species genetic exchange," says Deem. "We know that the majority of the DNA in the genomes of some animal and plant species - including humans, mice, wheat and corn - came from HGT insertions. For example, we can trace the development of the adaptive immune system in humans and other jointed vertebrates to a HGT insertion about 400 million years ago."
Could HGT that occurred in the distant past account for the incredible development of the human brain, and the subsequent human-chimp split? If so, what piece of vital genetic information did we acquire that gave us our cognitive edge? Equipped with a greater understanding of HGT, researchers at the University of California (UC) and the Department of Statistics have been able to pinpoint one of the fastest evolving pieces of DNA in the human genome, which just happens to be linked to brain development. Interestingly enough, this chunk of DNA - known as "Human Accelerated Region 1" or HAR1 - also happens to have appeared solely in humans around the time of the human-chimp split, and according to researchers, it's a real standout. "It's evolving incredibly rapidly," says UC's Katie Pollard. "It's really an extreme case." According to Pollard, only 2 changes out of 118 in HAR1 separate chimps and chickens, compared with 18 changes in humans over the 5 million year period since we shared a common ancestor with our bird-brained relatives.
So, is it possible that a virus or bacteria helped slot in a vital gene that led to a rapid and dramatic disparity in cognitive levels, leaving our chimp cousins behind? These recent findings certainly seem to support studies conducted a number of years ago, which identified the remnants of ancient germ line infections called human endogenous retroviruses that make up a substantial part of the human genome. The study showed that these were evidence of volatile transpositional activity, or HGT, via viruses that occurred at the same time humans and chimps are believed to have diverged from a common ancestor - 6 million years ago. Back in 2004, University of Georgia's John McDonald remarked that: "There is a growing body of evidence that transposable elements have contributed to the evolution of genome structure and function in many species. Our results suggest that a bust of transposable element activity may well have contributed to the genetic changes that led to the emergence of the human species."
HGT certainly seems to be a viable candidate capable of explaining all manner of evolutionary mysteries, from rapid evolutionary change, to human cognition and consciousness, and perhaps, speculatively, even species change itself. The obvious question remains that if HGT is responsible for human brain development, could it happen again in another species? Deem concludes that it could, noting: "Life clearly evolved to store genetic information in a modular form, and to accept useful modules of genetic information from other species."