Principles of Human Evolution - Part 1

The Growth of the Evolutionary Perspective

In an introduction to the study of human evolution it is important to recognize the concept of continuity. Evolution is a continuous process that connects all living organisms. However, there are obvious gaps which exist, highlighting cases of discontinuity. The disparity between humans and other apes is one example of this. Humans are uniquely advanced, and yet are still products of the evolutionary process. This gap has been acknowledged first by Darwin in 1859 and four years later by Thomas Henry Huxley. After two great intellectual revolutions concerning humans place in the world, Copernicus declaring the earth was not in fact the center of the universe and Darwin’s publication of Origin, there waere palpable changes in conceptions of where humans should find themselves. Once thought to be separate from nature, after these revolutions humans could be viewed as being a part of nature. Early perceptions of these relationships were seen as a Great Chain of Being, a hierarchy of living organisms with humans at the progressive end.
Q: Even after 1859, humans were seen as naturally superior. Why didn’t the theory of evolution remove the ideal of the Great Chain of Being? How did perceptions of progress alter explanations of the inequality of races?
Q: Our textbook urges us to “mind the gap” concerning our place in evolutionary biology. What is meant by this metaphor?

Considering the historical views of human evolution, two principle matters have continued to dominate the debates over the years. One factor is how close humans are to other apes. Some have argued that humans and chimps are very close while others would like to emphasize the greater differences. The other factor involves how far back in time human characteristics go. The latter brings up issues of what makes us more human such as bipedalism and intelligence.
During the 1960’s scientists were prominent believers in the single-species hypothesis, stating that only one species of hominin could exist at any one time therefore publicizing the idea that human evolution was a single linear progression. Arguments placed great emphasis on the importance of culture. Because humans are characterized by exhibition of cultural traits, it would be impossible for more than one species to survive together. This hypothesis was dismissed a decade later given the current discoveries of fossils proving the coexistence of both Homo erectus and Australopithecus boisei. There are now two complex questions still under research: the origin of the human lineage and the origin of human characteristics.
Q: How have other arguments over the relationship between humans and apes changed over the last 150 years?

Despite differences in theoretical approaches, anthropologists have kept four key events in mind for explaining the development of modern humans. These include the origins of terrestriality, bipedality, encephalization, and culture and have been placed in various sequences depending on differing schools of thought. An important element of the evolutionary perspective, no matter the sequence, is the way in which this information is disseminated. According to anthropologist Misia Landau, evolutionary explanations have typically followed a narrative form by telling a hero’s story (as explained in the previous post). This is notably reflective of how scientists can be influenced by their biased worldviews concerning natural relationships and humans.
Q: “What binds together the narrative of the folk tale and the narrative of paleoanthropology is that both are events that occur through time, and so attracts the form of explanation where one event is causally related to another.” Rather than using storytelling, what would make for a better evolutionary explanation?

The Principles of Evolutionary Theory

Though the idea of biological transformation had been introduced prior to the publication of The Origin of Species, Darwin was the one able to back up his theory with extensive documentation and explanation. Since then, the theory of evolution has gone though many changes, while even today scientists debate the details.

Simply put, evolution is change over time, and natural selection (the survival of genes from one generation to the next) is the mechanism for this change. For natural selection to occur four conditions must be met. Organisms must reproduce offspring, resemble their parents through inheritance, exhibit variation within the population, and compete successfully for resources. As natural selection operates there are two possible consequences in the outcome. If natural selection stabilizes by favoring existing conditions then adaptation occurs. “The better fitted an individual is to its environment, then the better adapted it is (31).” In order for survival to take place, an organism must have helpful adaptive traits. Conversely, if natural selection needs to change into a new mutant form, then evolution occurs. Evolution is basically the change in generational gene frequencies.

Despite this simplicity, arguments arise in evolutionary explanation. This may be due to researchers asking different kinds of questions. Typical explanations for evolutionary events may include factors of conditions, causes, constraints, or consequences, the “four C’s of evolution.”
Q: Why are there different types of evolutionary explanation? How does each approach explain evolution differently?

Darwin’s original concepts included descent with modification coupled with natural selection. Demonstrating how natural selection occurred was not fully explained until the 1930’s, however. At this time new theories developed known as Neo-Darwinism, or Modern Synthesis. This model incorporated Mendelian genetics with a fuller understanding of inheritance. In the 1860’s Gregor Mendel discovered the basic workings of genes by experimenting on pea plants. He was able to conclude that physical traits carry genes that are passed down to subsequent generations intact (without blending). It soon became apparent that phenotypes are influenced by several genes depending on which ones are dominant or recessive, though “no simple relationship exists between a mutation and the degree of phenotypic change it might produce (39).”
Q: What are the foundational differences between Darwinism and Neo-Darwinism? How have new understandings of population genetics changed evolutionary theory?

Other aspects of selection also occur. “Where natural selection adapts an organism for survival in its environment, sexual selection adapts it to the needs of obtaining a mate (42).” This can be seen at the individual level as species selfishly compete among themselves in order to reproduce. However, altruism can arguably exist as well when animals are seen to work for others giving up their own chances at mating. Selection is therefore present at the group level allowing for the social group to succeed rather than the individual. Different levels have demonstrated how we have advanced our understandings of the workings of adaptation.
Q: What are other levels of selection seen in nature?

Pattern and Process in Evolution

This chapter addresses modern debates in evolutionary theory, most notably the question of whether natural selection explains all aspects of change or whether there are other possible mechanisms at work. Basically, how do we explain change? In order to study the patterns of change, it is relevant to comprehend speciation. Given the multiple definitions of a species, due to the complexity of explaining continuity over time and the demarcation of where one species starts and the next begins, it is important to know how a new species arises. One explanation is anagenesis, which occurs through evolution of one species slowly over time. A new species arises when the descendants become so different they no longer resemble the original organism. Another possibility is cladogenesis. This occurs when a species becomes isolated and evolves separately on its own in accordance to the environment it is subjected to. Islands, mountain ranges, and oceans are all barriers which may split species. Because smaller populations have less variation, they are susceptible to faster changes. The latter event is known to create adaptive radiation, the branching that occurs after an evolutionary novelty like feathered flight or bipedalism.

The aforementioned Neo-Darwinian theory sees speciation at the micro-level, basically that natural selection is a slow gradual process of change over time. The gap in the fossil record, the fact that few transitional forms have been found, has led to more recent macro-level explanations for change. The punctuated equilibrium theory proposed in part by our favorite paleontologist Steven Jay Gould would disagree with gradualism stating that species remain relatively constant over long periods of time and when change comes it happens very fast. What is of interest within macroevolution theories is the study of trends within groups of species such the increase of brain size during human evolution. Within this theoretical framework lies the concept of species selection; due to static species over time traits must evolve in a direction within the clade and not the individual species. Extinction is most likely to occur for species with unique environments and low adaptation capabilities, and selection could be happening at the group level. All debates about evolutionary patterning are imperative for understanding “the context in which human evolution must be assessed and explained (55).” What is agreed upon is that linear progression did not occur and the tree of life is very bushy for each species.
Q: Would you rather argue for gradualism or punctuated equilibrium?

Another important element when studying evolutionary theory involves looking at what influences change. Three motives have been recognized, the first being inherited factors of a lineage, second concerning the interactions between species and groups (competition), and thirdly the natural environment. The second point, “biotic context” is interesting when it implies that change can exist purely based on behavior.
Q: Can evolution occur without environmental factors? Can you give examples of struggle among groups and within species that could lead to change?

Environment, it can agreed upon, does influence the survival of organisms. Cladogenesis may take place through either topographic or climatic alterations, both of which are effected by plate tectonics. The environment shifts constantly due to the fluidity of the earth’s crust and the plates that it rests upon. Due to this fluctuation landmasses have become isolated along with the plants and animals dependant of its resources. Either organisms evolve or they die out and become extinct. Plate tectonics have been a major justification for the explanation of species’ origins. Because of the barriers they create there are greater chances for the diversity of life. This has led to the habitat hypothesis stating “species’ response to climate change represent the principal engine of evolutionary change (65).” Evolutionary geography models have also changed over time becoming much more complex.
Q: How may climate change and habitat have affected hominin evolution?
Q: According to the Turnover-Pulse hypothesis, because environmental factors are so important, is it possible for a major climatic event to cause change across multiple species?

Another pattern worth pursuing is how much of a role extinction plays in evolution. Now seen as an important part of the process, it tends to be linked with environmental change. After the Cambrian explosion, close to 30 billion species have evolved on earth. With only 30 million in existence today it is clear that extinction is important to study. At one time catastrophism was the popular worldview thinking that “individual events that wiped out all of existing life, setting the stage for new waves of creation (75),” was the reasoning. Charles Lyell, however, argued for uniformitarianism, believing instead that slow forces were at work over long periods of time. Though the latter won out the debates, it is still clear that catastrophes must have occurred given geological evidence (extinction of the dinosaurs). It may be beneficial to view extinction as a combination of bad luck and selection when determining why some species survive and others do not.
Q: What attributes could help a species chance of surviving?
Q: Why should the study of mass extinction be a significant factor in the study of evolution?

The Geological Context

In looking at human evolution most evidence is found in the geological record. Therefore a firm understanding of earth science is also required. In conjunction with this is the need to establish accurate dating methods. Without dates much of what is found is irrelevant in search for answers about our human past. For fossils, direct dating methods on the material itself do not exist and employing other modes for finding out how old something is is possible. Scientists use both relative and absolute dating techniques in this quest, which we have discussed in the previous class.

Human history rests on sources from fossil and archaeological contexts. Taphonomy, which studies the intricacies of burial and fossil formations, tells us that many deposits are difficult to interpret due to multifarious interruptions such as scavenger raids. Typically, the only bones to remain intact are the strongest ones like jawbones and teeth. What is most important is that the remains be buried as soon as possible. This raises the chances that a specimen might be fossilized (transformation into the lithosphere) and discovered later. Taphonomists have identified most remains as a result of shifting water from rivers and lakebeds and have even located bones in ancient caves.
Much debate revolves around whether deposits result from human, animal, or geological activity. Cut marks on bones may imply carnivorous activity, scavenger commotion, trampling from later herds, or stone tool use. However difficult it may be, taphonomic approaches are crucial to understanding environment, diet, and even culture thousands of years ago.
Q: What other examples are there in distinguishing human behavior from animal activity in the fossil and archaeological record?