A chimpanzee enters a room where food is hidden in one of two opaque containers. A human gazes at the container that hides the food. Reaches for it with outstretched arm. Marks the container with a wooden block. The chimp doesn't get the message, even though chimpanzees are one of Homo sapiens' two closest extant primate relatives and might be expected to figure it out. Biological anthropologist Brian Hare and colleagues tried this game with 11 chimps, and only two of the brainy apes used the conspicuous cues to find the food.
Dog owners may not be surprised to learn that nine of 11 dogs in the same situation correctly read the human signals and found the food. A control exercise established that odor was not a cue in either trial.
"Our new work provides direct evidence that dogs' lengthy contact with humans has served as a selection factor, leading to distinct evolutionary changes," says Hare, who recently completed his Ph.D. in anthropology in Harvard's Faculty of Arts and Sciences. "This is the first demonstration that humans play an ongoing role in the evolution of canine cognition."
Ádám Miklósi led a group of researchers at Eötvös University in Budapest, Hungary who conducted the "shell game" tests on wolves. The test wolves were raised by humans and socialized to a comparable level as their dog counterparts. But although they could follow some signals, the wolves could not perform to the level of dogs.
Miklósi's test also included an important second step. He presented the animals with an unsolvable problem—a bowl of food that was impossible to access. The team found that while wolves continued to work at the unsolvable problem for long periods, dogs quickly looked at the humans for help.
Dec. 4 — The Eves of the dog world are five or six wolf females that lived in or near China nearly 15,000 years ago, according to a series of genetic research.
The researchers believe that by 10,000 to 12,000 years later, 10 "progenitor breeds" of dog had been created to fulfill different roles alongside their masters. It took a further 5000 to 3000 years for people to create the 300 or so pure breeds known today.
What is interesting about this result from a human evolutionary perspective is that it demonstrates how, contrary to popular belief, 10,000 or 20,000 years of selective pressure from relatively new environmental factors can produce large changes in shape, cognitive function, and behavior of a species. The example of dogs changing so much under human influence suggests the possibility that humans have changed a great deal as they moved out of Africa and evolved to fit into various ecological niches around the world.
An example of an evolutionary adaptation in humans that may have developed as recently as dogs developed from wolves occurred in the human Andean population which developed an adaptation to high altitudes.
Previous studies have shown that the Tibetan, Ethiopian and Andean populations have developed slightly different ways of boosting their oxygen levels to cope with the thin air. Those in the Andes pump out more haemoglobin - a molecule that carries oxygen around in the blood. The Tibetans, by contrast, have relatively low haemoglobin levels but breathe faster to take in more oxygen. "The slightest bit of exercise makes them really pant," Beall says.
The Tibetans probably had more time in which to develop high altitude adaptations and certainly the Ethiopians had more time since humans have been in Africa for a longer period of time. But the Andean human adaptation couldn't begin until the human populations came across the Bering Strait and then migrated all the way to South America.
Loren Cordain claims that the ability of adult northern Europeans to digest lactose sugar is a fairly recent adaptation that may have become widespread in just the last few hundred generations of humans.
Commentary: There are calculations which estimate how long it took to increase the gene for adult lactase persistence (ALP) in northern Europeans from a pre-agricultural incidence rate of 5% to its present rate of approximately 70% [Aoki 1991]. (Note: The enzyme lactase is required to digest the sugar lactose in milk, and normally is not produced in significant quantity in human beings after weaning.) In order for the gene frequency to increase from 0.05 to 0.70 within the 250 generations which have occurred since the advent of dairying, a selective advantage in excess of 5% may have been required [Aoki 1991].
Therefore, some genetic changes can occur quite rapidly, particularly in polymorphic genes (those with more than one variant of the gene already in existence) with wide variability in their phenotypic expression. ("Phenotypic expression" means the physical characteristic(s) which a gene produces.) Because humans normally maintain lactase activity in their guts until weaning (approximately 4 years of age in modern-day hunter-gatherers), the type of genetic change (neoteny) required for adult lactase maintenance can occur quite rapidly if there is sufficient selective pressure. Maintenance of childlike genetic characteristics (neoteny) is what occurred with the geologically rapid domestication of the dog during the late Pleistocene and Mesolithic [Budiansky 1992].
Influence of human culture on genetic selection pressures. However--and this is where it gets interesting--those population groups that do retain the ability to produce lactase and digest milk into adulthood are those descended from the very people who first began domesticating animals for milking during the Neolithic period several thousand years ago. (The earliest milking populations in Europe, Asia, and Africa began the practice probably around 4,000 B.C.) And even more interestingly, in population groups where cultural changes have created "selection pressure" for adapting to certain behavior--such as drinking milk in this case--the rate of genetic adaptation to such changes significantly increases. In this case, the time span for widespread prevalence of the gene for lactose tolerance within milking population groups has been estimated at approximately 1,150 years--a very short span of time in evolutionary terms.
It is worth noting that domestication of milk animals was such a large selective advantage that it could cause the mutation for lactase expression in adults to be selected for in a relatively short period of time. But since the selective pressure for adult lactase expression was very strong this suggests than any kind of behavior or other aspect of human physiology that was beneficial for doing milk animal herding and protection of milk animals would also have been selected for very strongly at the same time that adult lactase expression was being selected for. We have to consider the possibility that personality types more suited for the herding-tending and herd-protection may have been fundamentally different than the personality types most suited for a hunter-gatherer lifestyle that involved no use of milk animals.
Another post-Africa adaptation in humans is the spread of a mitochondrial mutation for generating more heat in colder weather.
These lineages are not found at all in Africans but occur in 14 percent of people in temperate zones and in 75 percent of those inhabiting Arctic zones. Wallace and his colleagues say this correlation is evidence that the lineages were positively selected because they help the body generate more heat.
Wallace says that climatic selection may have operated on the human population from the moment it moved north of the African tropics. Most such pioneers died but two lineages, known as M and N, arose in northeast Africa some 65,000 years ago and might have been adapted to temperate climates. Almost everyone outside of sub-Saharan Africa has mitochondria descended from the M and N lineages.
The writers of the research paper reporting on the heat-generating mtDNA variation speculate that human mtDNA has adaptations for local environmental conditions that are making humans have higher incidences of a number of diseases due to modern environments and diets.
Evidence has already accumulated that different human mtDNA lineages are functionally different. Haplogroup T is associated with reduced sperm motility in European males (30), and the tRNAGln nucleotide position 4336 variant in haplogroup H is associated with late-onset Alzheimer's disease (31). Moreover, Europeans harboring the mild ND6 nucleotide position 14484 and ND4L nucleotide position 10663 Leber's hereditary optic neuropathy missense mutations are more prone to blindness if they also harbor the mtDNA haplogroup J (32, 33), and haplogroup J is associated with increased European longevity (34). Because haplogroup J mtDNAs harbor two missense mutations in complex I genes (Y304H in ND1 and A458T in ND5), in addition to the above-mentioned L236T variant in the cytb gene, these polymorphisms all could affect the efficiency of OXPHOS ATP production and thus exacerbate the energy defects of mildly deleterious new mutations.
Given that mtDNA lineages are functionally different, it follows that the same variants that are advantageous in one climatic and dietary environment might be maladaptive when these individuals are placed in a different environment. Hence, ancient regionally beneficial mtDNA variants could be contributing to modern bioenergetic disorders such as obesity, diabetes, hypertension, cardiovascular disease, and neurodegenerative diseases as people move to new regions and adopt new lifestyles.
In humans mitochondrial DNA (mtDNA) is only 16,569 DNA letters long whereas the DNA in the human cell nucleus is over 3 billion letters long. Note that while the mtDNA is very small it still manages to have many variations with different effects on disease risks and environmental adaptation. It seems likely that the mtDNA heat variation is not the only mtDNA variation is a result of selective pressures to allow humans to adapt better to local conditions.
Another important thing to note about canine evolution is that to the extent that dog breeds developed special adaptations to perform various functions those dogs reduced the need for humans to do those functions and hence changed the selective pressure on humans.
"We know that dogs were useful for lots of things in Stone Age culture, as draft animals, in hunting, for warmth, and for protection," said Jennifer Leonard, a postdoctoral fellow at the Smithsonian Institution’s National Museum of Natural History. And in sharing food, shelter, survival and play, modem dogs have somehow genetically acquired an insight about humans that has earned them the title of man's best friend
For instance, a hunting dog that could smell prey reduced the need for humans to have an acute sense of smell for that purpose. Therefore the domestication of dogs must have changed the selective pressures on humans. Those changes in selective pressures must have been different depending on the types of dogs and the ecological niches various human groups found themselves in. Human groups that learned to train and work with dogs for various purposes had a selective advantage against human groups that did not do so. So just as humans have exerted selective pressures in dog evolution it seems highly likely that dogs have caused selective pressures in human evolution.
|Share |||Randall Parker, 2004 February 20 08:57 PM Trends, Human Evolution|