Human Evolution

Introduction

All living things are shaped over time by natural selection, which is the appearance of new characteristics by random mutation and the preservation of favorable ones in the struggle to survive and reproduce. All life has been shaped by natural selection, including human beings, who arose from a complex, branching ancestral tree of primate ancestors over the last several million years. Recently, scientists have suggested that changes in the ancient African climate may have triggered crucial events in human evolution, such as the development of bipedal (two-footed, upright) walking, by placing selective pressure on our ancestors—that is, by creating conditions favoring characteristics that had not previously been needed. Some have also suggested that other, separate climate changes about 100,000 years ago and 40,000 years ago further shifted our ancestors toward the modern human form.

Historical Background and Scientific Foundations

The first people to explain evolution by means of natural selection were the British biologists Charles Darwin (1809-1882), whose On the Origin of Species (1859) is considered one of the most important works in the history of science, and Alfred Russel Wallace (1823-1913), who discovered the principle independently. Although scientifically controversial at first, evolution won scientific acceptance in the early twentieth century when the new science of genetics was wedded to the older ideas of Darwin. Although it is called a “theory,” in scientific speech this means simply a body of explanations, not an idea that is uncertain or tentative. Evolution is a fact; evolutionary theory seeks to explain exactly how it happened.

Some of the most interesting questions for evolutionary theory center on the appearance of our own species, Homo sapiens, which probably evolved in the Great Rift Valley of eastern Africa between 2.6 million and 1.7 million years ago. Since the 1980s, scientists have suggested that climate-related changes in the environment drove evolutionary events in human history, including the appearance of bipedal walking, larger brains, and behavioral adaptability, as well as the first migrations from Africa to other continents.

Geological evidence shows that from 12 to 1.5 million years ago, mountain-building in eastern Africa changed the region from a flat area covered by rainforest to a highly diverse realm of mountains, valleys, and plateaus covered by many kinds of vegetation. This has led some to suggest the “savanna hypothesis”—the view that humankind's ability to walk on two legs evolved in response to the changing African landscape. If this view is correct, upright walking freed one's hands to manipulate tools, which encouraged the evolution of larger brains.

There are other ways in which climate change may have influenced our early evolution. Records of global climate derived from layered muck deposits from the ocean floor show that about 3 million years ago, the global climate became cooler and more variable than it had been. Glaciers formed in the northern hemisphere and the genus Homo—the group of species of which we are a part—appeared. Three unusually wet periods occurred between 2.7 million and 1 million years ago. According to British scientist Mark Maslin, speaking in 2006, “These temporary humid periods would have imposed huge impacts on early humans, and our research [showing the occurrence of the wet periods] provides strong evidence for theories in which early human species evolved and spread out in response to a rapidly changing environment.”

WORDS TO KNOW

BEHAVIORAL ADAPTABILITY: Capacity of an organism or society to survive altered environmental conditions by changing one or more behaviors. Changed behaviors might include timing of food-seeking, mating, egg-laying, and resting; use of alternative food resources, altered economic activities or building techniques; and many more.

BIPEDAL WALKING: Walking on two legs. The evolution of bipedal walking in early human ancestors freed up the hands for gripping and carrying, which may have triggered evolution of the enlarged human brain.

DROUGHT: A prolonged and abnormal shortage of rain.

EVOLUTION: Biological change. The process by which all life forms have developed. In evolution, slight, inheritable variations occur in each new generation of plant, animal, or microorganism. A few of these random variations (mutations) happen to be helpful; individuals bearing these variations leave more offspring. Gradually, even the most complex structures, such as the brain or eye, can be produced by the evolutionary process of random variation combined with natural selection. In geological terms, evolution means change over time.

MEGADROUGHT: Any unusually widespread, severe, and long-lasting drought. The term is generally reserved for droughts covering entire subcontinental regions and lasting for many years.

NATURAL SELECTION: Primary mechanism shaping biological evolution. Individual organisms that have inherited variations of form that help them to survive and thrive are naturally selected to leave more offspring: later generations will therefore be dominated by individuals inheriting the same variation. Accumulation of such changes over millions of years can produce new species and complex new structures such as the vertebrate eye.

SAVANNA HYPOTHESIS: Educated scientific guess (hypothesis) that human upright posture evolved in response to mountain-building in eastern Africa 2 to 1.5 million years ago, which changed regional climate and replaced rainforest with a diverse ecosystem dominated by savanna (grassy plain with widely spaced trees).

Impacts and Issues

Climate and evolution have interacted with each other throughout Earth's history. The evolution of green algae changed Earth's atmosphere, releasing oxygen and making the evolution of multicellular life possible. Human beings, who may have evolved because of past climate changes, are changing the global climate today. But the ancient climate and our own evolution is not clear and unambiguous. Our best climate records for the period in which Homo evolved are from ocean sediments and so reflect global climate, not regional African climate. Marine-sediment cores from deep African lakes, which were only beginning to be obtained in the early 2000s, may give a more detailed picture of African climate during the early evolution of our species.

In 2007, researchers announced that sediment cores from Lake Malawi had provided a high-resolution picture of southeast African climate over the last 1 million years. They found evidence of a surprisingly severe drought from 135,000 to 75,000 years ago, during which the lake shrank by 95%. About 70,000 years ago, the regional climate became wetter and more stable. Shortly thereafter, according to other evidence, human populations greatly increased in size and a small founder group left Africa to become the ancestors of all modern non-Africans. It is not yet known whether or not the human race suffered a true population bottleneck during the mega drought, a time when there were only a few living individuals.

Primary Source Connection

Climate change is a contributing factor to evolutionary changes in plants and animals, including humans. This article from the journal Science tells how an examination of rock sequences could determine whether climate changes that took place millions of years ago occurred on a local, basin, continental, or world scale. Understanding the nature of climate change that occurred long ago may give scientists a better understanding of the conditions that gave rise to the genus Homo.

CLIMATE CHANGE AND HUMAN EVOLUTION

There is debate about the precise time and place of the origin of the genus Homo, with time estimates ranging from 2.6 to 1.7 Ma. Synchrony of the climatic signal and the evolutionary event thus remains in question. The related notion that fluctuating lake levels provided environmental stress that drove speciation does not provide a mechanism for how this could have exerted selective pressure on the immediate ancestor of Homo and resulted in the emergence of a new genus and species ….

The way forward is to carefully match the quality and scale of the data with the scale of the question. We cannot expect to link global- or continental-scale climate processes to major events and trends in human evolution without first disentangling basin- and regional-scale environmental signals in strata that contain the hominin fossil record. This complexity can work in our favor, however. A multibasin, multiproxy approach is now possible because paleoclimate data and chonostratigraphic correlations are becoming available for a large number of rock sequences. If many stratigraphic sequences and independent climate proxies show similar, synchronous environmental shifts, this would be strong evidence for climate change affecting large regions of a continent, particularly when such trends can be matched to the marine core data. On the other hand, if basins show independent patterns of environmental change, this implies that they were locally buffered against larger-scale climate forcing, or that the available evidence is not sufficient to resolve small- versus large-scale environmental processes. The hominin fossil and cultural record could be reconsidered in light of such paleoclimatic meta-data sets. The strength of this approach depends on the number of sample points (that is, different basins and regions), accurate interpretations of climatic proxies, well-resolved correlations between basins, and a healthy dose of devil's advocacy before making a leap to global-scale interpretations. It is also worth remembering that climate was only one of many factors affecting human evolution; biological processes including genetic innovation, interspecies competition, and dispersal ability also could have played defining roles.

Rather than a simple story of global climate drumbeat and evolutionary response, more informative and exciting revelations about the 7-million-year development of hominin morphology, behavior, and culture will likely come from detailing the prolonged tension between local ecosystems and global climate change. This is also a strikingly relevant theme for the future of our species.

Anna K. Behrensmeyer

behrensmeyer, anna k. “climate change and human evolution.” science 311 (2006): 476-477.

See Also Adaptation; Climate Change.

BIBLIOGRAPHY

Periodicals

Behrensmeyer, Anna K. “Climate Change and Human Evolution.” Science 311 (2006): 476-478.

Scholz, Christopher A., et al. “East African Megadroughts Between 135 and 75 Thousand Years Ago and Bearing on Early-Modern Human Origins.” Proceedings of the National Academy of Sciences (September 4, 2007).

Web Sites

Maslin, Mark, et al. “A Changing Climate for Human Evolution.” Geotimes, September 2006. <http://www.agiweb.org/geotimes/sept05/feature_humanclimateevolution.html> (accessed October 26, 2007).

Owen, James. “Did Climate Change Drive Human Evolution?” National Geographic News, February 2, 2006. <http://news.nationalgeographic.com/news/2006/02/0202_060202_evolution.html> (accessed October 26, 2007).

Larry Gilman