EVIDENCE FOR EVOLUTION
At the heart of evolutionary theory is the basic idea that life has existed for billions of years and has changed over time. Overwhelming evidence supports this fact. Evidence for evolution comes from many different areas of biology, including: fossils, anatomy, molecular biology and direct observation.
Fossils are the preserved remains of previously living organisms or their traces, dating from the distant past. The fossil record provides snapshots of the past that, when assembled, illustrate a huge picture of evolutionary change over the past four billion years. This picture may be smudged in places and may have bits missing, but fossil evidence clearly shows that life is old and has changed over time.
One of the problems with the fossil record is that it contains gaps. most organisms never fossilize, and even the organisms that do fossilize are rarely found by humans. Nonetheless, the fossils that have been collected document the existence of now-extinct past species that are related to present-day species.
One of the few animals for which we have a fairly complete evolutionary record is the horse, where all the main stages of the evolution of the horse have been preserved in fossil form (see right).
If two or more species share a unique physical feature, such as a complex bone structure or a body plan, they may all have inherited this feature from a common ancestor. Physical features shared due to evolutionary history (a common ancestor) are said to be homologous.
For example, the forelimbs of whales, humans, birds, and dogs look very different on the outside. That's because they're adapted to function in different environments. However, if you look at the bone structure of the forelimbs, you'll find that the pattern of bones is very similar across species. It's unlikely that such similar structures would have evolved independently in each species, and more likely that the basic layout of bones was already present in a common ancestor of whales, humans, dogs, and birds.
DNA and the genetic code reflect the shared ancestry of life. DNA comparisons can show how related species are.
At the most basic level, all living organisms share:
- The same genetic material (DNA)
- The same, or highly similar, genetic codes
- The same basic process of gene expression (transcription and translation)
- The same molecular building blocks, such as amino acids
These shared features suggest that all living things are descended from a common ancestor, and that this ancestor had DNA as its genetic material.
In some cases, the evidence for evolution is that we can see it taking place around us! We can directly observe rapid, small-scale evolution in organisms with short lifecycles, such as the bacterium E. coli.
Sometimes mutations occur in E. coli’s DNA, though most of the time this causes the death of the cell. But occasionally, the mutation is beneficial to the bacteria. For example, it may allow resistance to an antibiotic, preventing the antibiotic from killing it. When that antibiotic is present, the resistant bacteria survive whereas the non-resistant bacteria die. Over time, the amount of antibiotic-resistant E. coli increase due to natural selection. Unfortunately, the emergence of antibiotic-resistant bacteria (not just E. coli) is becoming a growing problem in hospitals.