Genealogists use genetic information, oral histories and written records in order to trace bloodlines and create family trees that demonstrate kinship between individuals. Birth, death and marriage records, combined with oral histories and interviews of living family members aid in the construction of pedigrees, or charts which document common ancestry.
Evolutionary biologists are genealogists of sorts who trace the common ancestry of all living organisms. Like human genealogists, evolutionary biologists use multiple lines of evidence to trace common ancestry.
Understanding the evolutionary relationships between organisms, or evolution by common descent in scientific terminology, is aided by clues from four major sources: biogeography, comparative anatomy, the fossil record and molecular biology.
Charles Darwin's 1859 publication of "On the Origin of Species by Means of Natural Selection" was an instant best-seller. He presented evidence that all living organisms were descended from a common ancestor or common ancestors. Darwin called his evolutionary theory "descent with modification" because it was based on the principle of common ancestry and subsequent evolutionary change. The mechanism he proposed for how evolution occurred was natural selection, often referred to as "survival of the fittest."
Evolution by natural selection, or common descent, is the foundation for all of modern biology. Principles from evolutionary theory are used in medicine, for understanding how we track and treat disease; in agriculture, for crop improvement and pest control; in the biofuels industry; in forensics and crime labs; in conservation planning and more.
The concept of evolutionary theory is widely misunderstood due to differences in language use between the scientific community and the general public. In the common vernacular, a theory is no more than an educated guess, at best. But theory has a specific meaning in the scientific community. A theory, in science, is an explanation for an observed phenomenon.
Other misconceptions surrounding common ancestry have to do with what the theory of evolution encompasses or claims. Evolution does not explain the origin of life; it explains what happens after the first life arose. Evolutionary theory also does not claim that humans evolved from apes, but rather that humans and apes once shared a common ancestor.