Biological evolution simply put is descent with modification. This definition encompasses small-scale evolution (changes in gene frequency in a population from one generation to the next) and large-scale evolution (the descent of different species from a common ancestor over many generations). Evolution helps us to understand the history of life.
Biological evolution is not simply a matter of change over time. Lots of things change over time: trees lose their leaves, mountain ranges rise and erode, but they aren’t examples of biological evolution because they don’t involve descent through genetic inheritance.
The central idea of biological evolution is that all life on Earth shares a common ancestor, just as you and your cousins share a common grandmother.
Through the process of descent with modification, the common ancestor of life on Earth gave rise to the fantastic diversity that we see documented in the fossil record and around us today. Evolution means that we’re all distant cousins: humans and oak trees, hummingbirds and whales.
Fundamental to the process is genetic variation upon which selective forces can act in order for evolution to occur. This section examines the mechanisms of evolution focusing on:
– Descent and the genetic differences that are heritable and passed on to the next generation;
-Mutation, migration (gene flow), genetic drift, and natural selection as mechanisms of change;
-The importance of genetic variation;
-The random nature of genetic drift and the effects of a reduction in genetic variation;
-How variation, differential reproduction, and heredity result in evolution by natural selection; and
-How different species can affect each other’s evolution through coevolution.
Adaptation is an important element of the evolution process. An adaptation is a feature that is common in a population because it provides some improved function. Adaptations are well fitted to their function and are produced by natural selection.
Adaptations can take many forms: a behavior that allows better evasion of predators, a protein that functions better at body temperature, or an anatomical feature that allows the organism to access a valuable new resource — all of these might be adaptations. Many of the things that impress us most in nature are thought to be adaptations.
Our species is an African one: Africa is where we first evolved, and where we have spent the majority of our time on Earth. The earliest fossils of recognizably modern Homo sapiens appear in the fossil record at Omo Kibish in Ethiopia, around 200,000 years ago. Although earlier fossils may be found over the coming years, this is our best understanding of when and approximately where we originated.
According to the genetic and paleontological record, we only started to leave Africa between 60,000 and 70,000 years ago. What set this in motion is uncertain, but we think it has something to do with major climatic shifts that were happening around that time—a sudden cooling in the Earth’s climate driven by the onset of one of the worst parts of the last Ice Age. This cold snap would have made life difficult for our African ancestors, and the genetic evidence points to a sharp reduction in population size around this time. In fact, the human population likely dropped to fewer than 10,000. We were holding on by a thread.
Once the climate started to improve, after 70,000 years ago, we came back from this near-extinction event. The population expanded, and some intrepid explorers ventured beyond Africa. The earliest people to colonize the Eurasian landmass likely did so across the Bab-al-Mandab Strait separating present-day Yemen from Djibouti. These early beachcombers expanded rapidly along the coast to India, and reached Southeast Asia and Australia by 50,000 years ago. The first great foray of our species beyond Africa had led us all the way across the globe.
Slightly later, a little after 50,000 years ago, a second group appears to have set out on an inland trek, leaving behind the certainties of life in the tropics to head out into the Middle East and southern Central Asia. From these base camps, they were poised to colonize the northern latitudes of Asia, Europe, and beyond.
Around 20,000 years ago a small group of these Asian hunters headed into the face of the storm, entering the East Asian Arctic during the Last Glacial Maximum. At this time the great ice sheets covering the far north had literally sucked up much of the Earth’s moisture in their vast expanses of white wasteland, dropping sea levels by more than 300 feet. This exposed a land bridge that connected the Old World to the New, joining Asia to the Americas. In crossing it, the hunters had made the final great leap of the human journey. By 15,000 years ago they had penetrated the land south of the ice, and within 1,000 years they had made it all the way to the tip of South America. Some may have even made the journey by sea.
The story doesn’t end there, of course. The rise of agriculture around 10,000 years ago—and the population explosion it created—has left a dramatic impact on the human gene pool. The rise of empires, the astounding oceangoing voyages of the Polynesians, even the extraordinary increase in global migration over the past 500 years could all leave traces in our DNA.
Reference: University of California Museum of Paleontology’s Understanding Evolution; University of Berkeley Evolution Edu Department; Meredith M (2011). Born in Africa: The Quest for the Origins of Human Life. New York: PublicAffairs; National Geographic ‘Human Journey’.