Evolution is one of the most critical elements of modern biological science, but it is arguably the most poorly-understood discipline of biology in the minds of the general populace. Most notably, the theory of common descent has raised objections from laypersons who resist the idea that all species alive today share a common ancestry.
The study of genetics, a science on the cutting-edge of technology today, may hold the key to helping the average person understand how evolution works and, even more fundamentally, that it really does happen. Even without the fossil record or anatomical studies of living organisms, genetics provides enough evidence for evolution to satisfy the most rigorous scientific review.
Any legitimate scientific theory must make testable predictions. If it is true that various living species share a common ancestor, then one would predict that the genomes--all the genes within an organism--of these species would bear notable similarities, and that the more recently two species diverged from that common ancestor, the more similar their genomes would be, since they would have had fewer generations over which to accumulate differences.
When geneticists examine the genomes of species believed to share a recent common ancestry, this is exactly what they observe. Endogenous retroviruses (ERVs) are a particularly conclusive example of shared genetic ancestry. ERVs are the result of a virus infecting an organism and inserting portions of viral genetic material into the DNA of the organism's reproductive cells; the modified DNA containing the viral mutation is then passed on to the offspring of the infected specimen. The retroviral DNA sequence does not code for anything, and so that particular sequence is highly unlikely to appear spontaneously.
If a given ERV gene sequence appears at the same place in the genomes of two different species, it strongly suggests a common ancestry for those two species, because that is the simplest way to explain that particular sequence of non-coding DNA appearing at that precise point in the genomes of multiple species. In practice, this phenomenon is observed thousands of times over in the human genome; there are approximately 200 thousand ERV sequences in the human genome, and less than one hundred of those are not shared by the species most closely related to humans, the chimpanzee.
Human evolution and common ancestry with other species is the facet of evolutionary biology held to be most controversial by the lay populace, but a closer look at human chromosomes reveals still further evidence for evolution from a shared ancestor with other primates. Humans have 23 pairs of chromosomes, while other great apes, such as chimpanzees, gorillas, and orangutans, possess 24 pairs. Though this would appear to suggest a significant genetic difference between humans and other apes, a deeper examination of chromosome 2 in humans suggests an alternate interpretation.
A chromosome contains a few discrete anatomical segments: at either end of a chromosome are non-coding sections of DNA called telomeres, and in roughly the middle of a chromosome strand is a segment called a centromere. Human chromosomes are no different, possessing telomeres and a centromere in the usual places, but chromosome 2 also contains the remnants of a second centromere, and telomere sequences at a point in the middle of the chromosome, as though two chromosomes fused together at their ends.
This impression is strengthened by the fact that chimpanzees possess genes identical to those found on chromosome 2 in humans, though they are present on two separate chromosomes in chimpanzees. The study of the human genome allows biologists to trace the genetic development of the human species, and reveals strong similarities to the evolutionary cousins of humans while showing how those genes have changed since the two species diverged.
There are many ways of tracking the progress of evolution in the past and observing the changes it causes in the present, but the study of genetics allows biologists to pinpoint specific mutations and modifications that have resulted in evolutionary change. When Charles Darwin first conceived of the theory of evolution by natural selection, he knew that some mechanism for heredity must exist, but he could not explain what it was or how it worked.
The modern study of genetics has given us the last piece of the puzzle, allowing us to understand mutation and inheritance, and providing us with both the explanation of, and the evidence for, evolution as we now define it: the change in frequencies of gene variations over time.