Modern Homo sapiens is still evolving. Despite the long-held view that natural selection has ceased to affect humans because almost everybody now lives long enough to have children, a new study of a contemporary Massachusetts population offers evidence of evolution still in action.
A team of scientists led by Yale University evolutionary biologist Stephen Stearns suggests that if the natural selection of fitter traits is no longer driven by survival, perhaps it owes to differences in women's fertility. "Variations in reproductive success still exist among humans, and therefore some traits related to fertility continue to be shaped by natural selection," Stearns says. That is, women who have more children are more likely to pass on certain traits to their progeny.(See the top 10 scientific discoveries of 2008.)
Stearns' team examined the vital statistics of 2,238 postmenopausal women participating in the Framingham Heart Study, which has tracked the medical histories of some 14,000 residents of Framingham, Mass., since 1948. Investigators searched for correlations between women's physical characteristics including height, weight, blood pressure and cholesterol levels and the number of offspring they produced. According to their findings, it was stout, slightly plump (but not obese) women who tended to have more children "Women with very low body fat don't ovulate," Stearns explains as did women with lower blood pressure and cholesterol levels. Using a sophisticated statistical analysis that controlled for any social or cultural factors that could impact childbearing, researchers determined that these characteristics were passed on genetically from mothers to daughters and granddaughters.
If these trends were to continue with no cultural changes in the town for the next 10 generations, by 2409 the average Framingham woman would be 2 cm (0.8 in) shorter, 1 kg (2.2 lb.) heavier, have a healthier heart, have her first child five months earlier and enter menopause 10 months later than a woman today, the study found. "That rate of evolution is slow but pretty similar to what we see in other plants and animals. Humans don't seem to be any exception," Stearns says.(See TIME's photo-essay "Happy 200th Darwin Day.")
Douglas Ewbank, a demographer at the University of Pennsylvania who undertook the statistical analysis for the study, which was published Oct. 21 in the Proceedings of the National Academy of Sciences (PNAS), says that because cultural factors tend to have a much more prominent impact than natural selection in the shaping of future generations, people tend to write off the effect of evolution. "Those changes we predict for 2409 could be wiped out by something as simple as a new school-lunch program. But whatever happens, it's likely that in 2409, Framingham women will be 2 cm shorter and 1 kg heavier than they would have been without natural selection. Evolution is a very slow process. We don't see it if we look at our grandparents, but it's there."
Other recent genetic research has backed up that notion. One study, published in PNAS in 2007 and led by John Hawks, an anthropologist at the University of Wisconsin at Madison, found that some 1,800 human gene variations had become widespread in recent generations because of their modern-day evolutionary benefits. Among those genetic changes, discovered by examining more than 3 million DNA variants in 269 individuals: mutations that allow people to digest milk or resist malaria and others that govern brain development.(Watch TIME's video "Darwin and Lincoln: Birthdays and Evolution.")
But not all evolutionary changes make inherent sense. Since the Industrial Revolution, modern humans have grown taller and stronger, so it's easy to assume that evolution is making humans fitter. But according to anthropologist Peter McAllister, author of Manthropology: the Science of Inadequate Modern Man, the contemporary male has evolved, at least physically, into "the sorriest cohort of masculine Homo sapiens to ever walk the planet." Thanks to genetic differences, an average Neanderthal woman, McAllister notes, could have whupped Arnold Schwarzenegger at his muscular peak in an arm-wrestling match. And prehistoric Australian Aborigines, who typically built up great strength in their joints and muscles through childhood and adolescence, could have easily beat Usain Bolt in a 100-m dash.
Steve Jones, an evolutionary biologist at University College London who has previously held that human evolution was nearing its end, says the Framingham study is indeed an important example of how natural selection still operates through inherited differences in reproductive ability. But Jones argues that variation in female fertility as measured in the Framingham study is a much less important factor in human evolution than differences in male fertility. Sperm hold a much higher chance of carrying an error or mutation than an egg, especially among older men. "While it used to be that men had many children in older age to many different women, now men tend to have only a few children at a younger age with one wife. The drop in the number of older fathers has had a major effect on the rate of mutation and has at least reduced the amount of new diversity the raw material of evolution. Darwin's machine has not stopped, but it surely has slowed greatly," Jones says.(See TIME's special report on the environment.)
Despite evidence that human evolution still functions, biologists concede that it's anyone's guess where it will take us from here. Artificial selection in the form of genetic medicine could push natural selection into obsolescence, but a lethal pandemic or other cataclysm could suddenly make natural selection central to the future of the species. Whatever happens, Jones says, it is worth remembering that Darwin's beautiful theory has suffered a long history of abuse. The bastard science of eugenics, he says, will haunt humanity as long as people are tempted to confuse evolution with improvement. "Uniquely in the living world, what makes humans what we are is in our minds, in our society, and not in our evolution," he says.See pictures of Earth from space.See the top 50 space moments since Sputnik.
The History Of Human Evolution Essay
The History of Human Evolution
By definition, human evolution is the development, both biological and cultural, of humans. Human ideologies of how the evolution of man came to be is determined by cultural beliefs that have been adopted by societies going back as far as the Upper Paleolithic era, some 40,000 years ago. Through the study of paleoanthropology, we have come to determine that a human is any member belonging to the species of Homo Sapiens. Paleoanthropologists, while studying the evolution of humans, identify and explain evolutionary changes that occur throughout time that aid in the development of the human species. It will be through the examination of human physical traits, human origins from pre-humans to modern humans, and major discoveries that we will be able to understand the history of human evolution.
The Hominidae, or hominids are a group of upright walking primates with relatively large brains. The only existing representative of this family is the Homo sapiens. We can declare that all humans are part of the hominid family, yet not all hominids can be considered humans. However all humans are primates; although humans have developed very distinct traits from its genetically similar primate, the chimpanzees, such as bipedalism, meaning walking on two legs. “Bipedalism seems to be one of the earliest of the major hominine characteristics to have evolved.”(Microsoft Encarta) Bipedalism enabled humans to develop specific physical traits to accommodate their upright posture, such as a specialized pelvis, hip and leg muscles, and an S-shaped vertebral column. These traits, specific to humans, can be detected in fossil records therefore making bipedalism the defining factor in the physical traits of the sub family Homininae.
Another physical characteristic associated with the evolution of humans is that of the brain size. Modern humans have a “braincase volume of between 79.3 and 91.5 cubic inches”.(Gallagher) Throughout the development of humans, one can notice that the brain has more than tripled in size. This augmentation may be related to behavioral patterns of the hominids with an increase in number and sophistication of stone tools and other artifacts. The art of tool making along with other learnt skills made it possible for hominids to heighten their ability to live in a range of different environments.
“The earliest hominine fossils show evidence of marked differences in body size, which may reflect a pattern of the different sexes in our early ancestors”.(Gallagher) Females tended to be smaller, weighing in at about 70 lbs and measuring 3 to 4 feet tall, while men were on average 5ft tall and weighing 150lbs. This drastic size difference between genders decreased through time, sometime after a million years ago. “The third major trend in hominine development is the gradual decrease in the size of the face and teeth.”(Microsoft Encarta) Unlike the apes (from whom...
Loading: Checking Spelling0%