Biological Anthropology/Unit 3: Human Evolution/Trends

Hominin Evolution
Paleoanthropologists are trying to answer a number of questions about human evolution.


 * Why did our earliest ancestors stand up?
 * How did our ancestors provision themselves?
 * Why did some species become extinct until only one species, Homo sapiens, was left?
 * When, where and why did modern humans evolve?
 * What was the role of the Neandertals?
 * What makes us human?

While answers have been provided, new research continues to change and hone the picture of the evolution of humanity. This section of the course is not intended to provide a complete review of paleoanthropological research, but merely to provide an overview of key finds and outline trends in hominin evolution. For more in depth information, please see the "For Further Exploration" at the end of this page.

You may want to start with a 15-minute video by Louise Leakey Digging for Humanity and for a little bit of fun, check out The Simpsons take on evolution, both biological and cultural!

Morphological Trends in Human Evolution
There are a number of trends in the evolution of the proto-hominins to modern Homo sapiens. These do not occur all at once, but over millions of years. In general, Again, not all of these traits occur at the same time and there is variation among the various hominin species, but all of these morphological characteristics occur in the evolutionary line of Homo sapiens. More details will be given about these traits in the sections on the hominins.
 * forward movement of the foramen magnum
 * canines reduce in size
 * molars increase in size
 * diastema disappears
 * increased cranial capacity
 * decreasing prognathism (forward jutting of the lower part of the face)
 * thinning of the bone
 * rounding of the skull

Bipedalism
For a long time, paleoanthropologists thought that large brains were the first hallmark of becoming human; however, research in the 20th century showed that habitual bipedalism, or upright walking, was the first morphological trait on the road to humanity. To explore the anatomical changes associated with bipedalism, visit Step by Step: The Evolution of Bipedalism hosted by the Department of Anthropology at The University of Texas-Austin. Make sure you read the complete introduction, including the theories on why our ancestors stood up, all the way through the anatomical adaptations and the conclusion.

Encephalization of the Brain
Encephalization of the brain refers to a couple of things: 1) the increase in brain size over time and 2) the size of the brain in relation to total body mass. The brain-size to body mass ratio does not change that much in the hominins. "The stature of A. afarensis varied from 3.3 to 5 feet (1 to 1.5 meteres) tall, while that of modern humans varies from approximately 4.5 to over 6 feet (1.4 to 1.8 meters)" (Campbell and Loy 2000: 285). However, while there is gradual increase in brain size throughout the australopithecine lineage, it is not until early Homo that there is a significant increase in cranial capacity, approximately a 20% increase over australopithecines. More significantly is the approximately 40% increase in brain size of Homo erectus/ergaster and the earlier Homo species. It is not just the size of the brain that is important. During this process of encephalization, there is also a rewiring of the brain that coincides with the emergence of material culture such as stone tools. It is not until this occurs that hominins leave Africa, enabled greatly by cultural advances.

Non-human primate brains are symmetrical as are the brains of early hominins. With the emergence of Homo we see the lateralization of the brain--it becomes asymmetrial (right brain, left brain). We know this from endocasts. Endocasts form when minerals replace brain matter inside the cranium during the fossilization process. These endocasts allow paleoanthropologists to study the cortical folds of the brain and compare it to modern humans. Based on endocasts, researchers determined that three areas of the brain began to change in Homo: the cerebellum, which handles learned motor activities, the limbic system, which processes motivation, emotion and social communication, and the cerebral cortex, which is responsible for sensory experiences. It is these changes that may have allowed the early members of our genus to develop cultural adaptations to environmental pressures.

Why did the brain change in early Homo?

The question that confronted paleoanthropologists was why the brain changed. Big brains have some disadvantages: So, for a large brain to become fixed in the Homo population, the advantages had to outweigh the disadvantages listed above.
 * it takes approximately 25-30% of a human's metabolic energy to run their brain
 * requires infants to be born prematurely resulting in a longer period of infant dependancy (the average infant brain is about 1/3 the size of an adult brain)
 * longer infant dependancy is an increased drain on maternal energy; the mother must have proper nutrition not only for herself but for the nursing infant
 * it has been suggested that larger brains have decreased the bipedal efficiency of females because they must have a wider pelvis and birth canal to give birth to a large brained infant

One possible explanation incorporates the interaction of three different variables: group size, complex subsistence patterns, and the nutritional value of meat (Campbell and Loy 2000: 318). Let's address group size first. Research suggests that brain size and size of social groups correlates positively among living primates, implying that big brains helped individuals keep track of such things as dominance hierarchies, allies, etc. Second, a big brain allows for primates to keep track of large subsistence territories and allows for omnivores to develop strategies for collecting a wide-variety of foodstuffs. Third, eating meat is a relatively easy way to get the nutrition needed to run a big brain, which, as mentioned above, in modern humans takes about 1/3 of our daily metabolic energy. The argument for this, the social brain hypothesis, is laid out by Robin Dunbar in this [psych.colorado.edu/~tito/sp03/7536/Dunbar_1998.pdf article]. Dunbar also claims that it was changes in the neocortex, a 2-4mm thick top layer of the cerebral hemispheres, that were critical in the "homininzation" (development of cognitive abilities) of our ancestors. Please read this short article on the evolution of human cognitive abilities.

Geology &amp; Environmental Background
The Miocene period (roughly 23-5 million years ago) was geologically active in Africa. This is the period of the adaptive radiation of the apes and a period of mountain buliding that led to the formation of the Great Africa Rift Valley (see ). With the emergence of the rift mountains, the rains that heretofore had moved across the continent from the Atlantic Ocean were blocked (refered to as a rain shadow), leading to the aridification of Eastern Africa ( and ). The savanna environment that evolved in Eastern Africa was and is a much more open environment than the forested environment of Western and Central Africa, leading to rise of new adaptations for plants and animals. It is in this newly emerging environment that hominin evolution takes off, although recent research indicates that proto-hominins lived in Western Africa.

Paleoclimatic data has been correlated with speciation events in hominin evolution, but it does not seem to account for all speciation events. Paleoanthropologists are still working to identify the selective pressures that resulted in the evolution of different hominin species. Nonetheless, the paleoclimatic data suggests the following: This data has a tendancy to make us think that hominin evolution was driven by environmental changes; however, the presence of Sahelanthropus tchadensis in West Africa forces paleoanthropologists to acknowledge the possibility that geologic, climatic, and environmental changes occuring in Africa during the Miocene, Pliocene and Pleistocene had little to do with the evolution of hominins.
 * Grasslands spread in Africa between 10-5 million years ago duing a cooling and drying phase. It is during this time frame that the common ancestor of African apes and humans lived. The common ancestor was more than likely a quadruped who was arboreal or at least spent a significant amount of time in the trees. In the middle of this period, approximately 7-6 million years ago, the first bipedal hominin emerged, Sahelanthropus tchadensis. Sahelanthropus and a few other early hominins are generally referred to as proto-hominins in recognition of their primitive, ape-like features.
 * In the mid-Pliocene period, 3-2 million years ago, yet another cooling and drying phase is correlated with the adaptive radiation of the hominins, including the emergence of the robust australopithecines and the genus Homo.
 * Near the beginning of the Pleistocene period, also referred to as the Ice Age, the environment continued to get drier. Open habitats spread in East Africa. During this period, Homo ergaster (Homo erectus) emerges and eventually leaves the African continent.

Explore Human Evolution in Print
Boyd, Robert and Joan B. Silk. 2009. How Humans Evolved, 5th edition. New York: W. W. Norton.

Campbell, Bernard G. and James D. Loy. 2000. Humankind Emerging, 8th edition. Boston: Allyn &amp; Bacon.

Johanson, Donald and Kate Wong. 2010. Lucy's Legacy: The Quest for Human Origins. New York: Harmony Books.

Stringer, Chris and Peter Andrew. 2006. The Complete World of Human Evolution. New York: Thames &amp; Hudson.

Tattersall, Ian. 2008. The Fossil Trail: How We Know What We Think We Know About Human Evolution. New York: Oxford University Press.

Explore Human Evolution on the Web
Becoming Human Talk Origins Fossil Hominids Hall of Human Origins Science Daily: Human Evolution News Rediscovering Biology:Unit 9 Human Evolution BBC: The Evolution of Man Human Evolution: The Fossil Evidence in 3d

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