The theory of evolution began when man first noted his distinct similarity to apes. The possibility of a common ancester with the modern apes had been discussed since the mid 1800’s, but it wasn’t until Charles Darwin published his book, On the Origin of Species in 1859, that the idea was considered legitimate. Since that time, fossil records have helped us to deliniate and define our evolutionary history and to describe the many human-like species that have lived on the earth over the last 8 million years.
Charles Darwin studied a population of finches that had been isolated on the Galapogos Islands, and described the theory of natural selection, whereby over time, each finch had changed in ways that made it more successful in a particular niche. He observed fourteen different species of finches, all who were very similar, except for the size and shapes of their beaks. He noted how each type of beak was shaped very specifically for the type of food the bird ate and explained that the changes were due to the natural selection of the characteristics in a species that made it more successful at obtaining food and at reproducing.
If a finch with a slightly longer beak was more efficient at eating termites from holes in trees, he would survive longer and reproduce more than his short beaked counterparts who ate termites. His offspring that inherited his slightly longer beak would also survive longer and reproduce more. Over time, the longer beaked finches would be more prolific than the shorter beaked version within that particular niche and the adaptation would flourish while the shorter beaked finches would die out.
Eventually, speciation occured (the development into reproductively separate species), as the termite eaters with longer beaks became more distinctly separate within their environment and became reproductively isolated. This gradual change of a segment of a population into reproductively different species, in response to an environmental pressure (competition) for survival factors is called adaptive radiation. Although Darwin mentioned nothing about human evolution in his first book, his science minded peers were able to understand how his theory applied to humans.
Initially there was much debate by the general population over the idea that humans could have evolved from the same lineage as an ape, but by 1870s, the scientific community had already come to the general conclusion that a fossil record would be found in Africa, where our great ape relatives were found. In the 1920’s, as predicted, Raymond Dart found bones from Australopithecus africanus, a hominid who lived between 2-3 million years ago in the Pliocene Era.
They were discovered in a cave in Africa. From the skeletal structure he was able to determine that the specimen was bipedal; a defining trait of humans, and the size of its braincase was between that of modern humans and apes. Since that time, paleoanthropologists have been able to trace the evolutionary history of the primates including hominids, for some 85 million years and can determine many facts about their lifestyle through the study of fossils and other artifacts (O’Neal, 2006).
There is not a complete record, and although paleoanthropologists have presented us with many pieces of the puzzle, we do not know exactly how man evolved into the species we are today. Some researchers have speculated that many of our developed traits were due to adaptations for an aboreal life. The premise of this idea is that as angiosperms became prevailant, an overlapping canopy developed that allowed cross movement from tree to tree. The canopy was a niche that had little competition and provided advantages for primates. Over time and through natural selection, primates took to the trees.
They developed opposable thumbs and short finger nailed, grasping hands for holding onto branches, binocular vision for depth perception, and more complex brains to be able to better deal with balance and complex navigation (Simons, 1992). The difficulty with this idea is determining why, since there are other aboreal animals without these characteristics, primates developed them. There had to be some other factors that added the pressure for choice of these characteristics in primates. A primary characteristic of hominids is bipedalism. We likely developed from a chimpanzee-like, knuckle walking ancestor.
The reason for the change from using all four limbs for locomotion to standing upright, on two limbs is unknown. There are several theories that focus on environmental conditions as being the motivating factor. One such theory is called the Savannah Hypothesis which proposes that due to drier conditions, the African forests thinned and disapeared. Bipedalism developed both as an adaptation to facilitate movement across the grasslands and as a way to give early hominids use of their hands for food cultivation and tool use since they were no longer needed for locomotion (Simons, 1992).
The pressures that fuel evolutionary trends are those that are required for the species improved survival. We immediately think of food and shelter, but perhaps the most critical in insuring the continuation of a species is reproductive strategy. Primate reproductive strategies today include those that are monogmous (one male and one female), those that are polygynous (one male and multiple females), those that are polandrous (one female, multiple males), and those that are promiscuous (multiple males, multiple females).
Eighty percent of primates practice polygyny, eighteen percent practice monogomy, with polandry seen only in some of the Callitrichids, the marmosets and tamarins, and promiscuity seen in chimps. The evolution of polygyny can be easily rationalized. In polygynous strategies, the egg has a high energy cost and the female puts her energy into childcare. Males may help with childcare as well, but females have a long period of gestation and then a period of lactation, so childcare is their primary focus.
Females must focus also on food, not only for themselves, but for their offspring, whether directly or as an energy source for lactation. Therefore, female primates will naturally search for food sources. It is advantageous for females to congregate for childcare, for shared food sources and for safety. Reproductive success for a female primate is through the focused care of each individual offspring. Males do not have the same enforced biological costs throughout the reproductive process. Sperm are easily produced in large quantities at a low energy cost.
Reproductive success for males is through the number off offspring he can produce. Under these circumstances, polygyny is the most efficient reproductive strategy for species survival (O’Neal, 2006). Mongomous primates have a different environment. Monogomy is driven by the spacial arrangements of females or when the male’s role in childcare is critical. When females do not gather, it is logistically not feasible for the male to obtain additional mates. Gibbon females will not tolerate another female in her claimed territory.
The males might have the tendancy toward polygyny, but the females won’t allow it. Monogomy in tamarins is necessary because of the relative size of the offspring to parent size. The young, although still dependent on parental care, quickly grow to near adult size. It is nearly impossible for a single female to raise her offspring alone. Although reproductive strategy is varied among primates, each species’ tendancy is based on the evolutionary pressure for their survival and success. Recent fossil discoveries have been able to describe an evolutionary history that dates back as far as 4. million years ago to the Autraliopitecines.
An almost complete skeleton was found in 1974 in Hadar, Ethiopia and adopted the name Lucy. This was the first human –like homonid. Lucy was bi pedal and had a large brain for her body size of 48 inches. Since that time, other fossils of Australopitecines have been uncovered, but Lucy remains the most complete, and is the standard for the species (Primate Info Net, 2008). Homo habilis is thought to have descended directly from a species of Australopithecine hominid and lived from approximately 2. 2 to 1. million years ago.
Fossils found in Tanzania in 1962 by Louis and Mary Leakey show that Homo habilis was short and had long arms, like the Australipithecines, but had a decrease in facial protrusion, giving it a more human-like appearance. They used rudimentary tools in their hunting and gathering (Primate Info Net, 2008). Homo erectus had a larger brain case than Homo habilis and stood slightly more erect. They also had more sophisticated tools that were able to cut and slice. They had discovered the use of fire and were the first actual hunter-gatherer society.
They lived approximately 2. 2 to 1. 1 million years ago and were the first to leave Africa and travel throughout the Great Rift Valley (Primate Info Net, 2008). . Homo sapiens archaic or Neanderthal Man inhabited Europe and parts of Asia approximately 1. 2 million years ago. They were a fairly sophisticated society. They buried their dead, had fire, ate meat and had modern tools like hand axes and knives. They were adapted to a colder climate with short, robust statures and large noses. Their brain capacity was greater than modern humans and they had language.
There were probably only about 10,000 Neanderthals and they slowly disappeared as modern man, Homo sapiens sapiens began to appear 200,000 years ago (Primate Info Net, 2008). . Throughout man’s evolutionary history there has been a trend toward greater intelligence, mobility and refined physical and mental control. We are evolving each day towards a better version of ourselves. Are we the final model? Or will we eventually be replaced by a more efficient, successful hominid? In all likelihood, we will.
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Topic: Primate Evolution
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