If any single theme really stands out in Evidence of Evolution, this is it. To acknowledge that evolution has occurred is to acknowledge that living things are united by common ancestry, and that they have diversified over time to meet the specific demands of widely variable environments.
The great diversity of life is easy to see, but the unity (common ancestry) of life can be a little harder to wrap your mind around. We mentioned it a bunch of times earlier, and discussed how various forms of evidence support the notion of common ancestry, but let's explore this idea in depth. What, exactly do we mean by common ancestry? Common ancestry means that all living things—past, present, and future—share an ancestor, and all descended from that one individual. Scientists believe that the last (most recent) common ancestor to all existing life forms lived about 3.9 billion years ago. This ancestor gave rise to the three domains of life—Eukarya, Bacteria, and Archaea—and all the organisms that comprise those domains.
You're probably thinking this is hard to believe, considering the vast differences between E. coli bacteria, the mushroom on your pizza, and humans. But look a little more closely; they all rely on nucleic acids to encode information, they all synthesize proteins by similar processes, and for all of them, the basic unit of life is the cell. The differences between these organisms have evolved in the billions of years since their common ancestor lived. In other words, since that ancestor lived so long ago, there has been plenty of time for evolution to shape these organisms into the disparate forms they take today.
It's also important to note that, while the ancestor for ALL life forms lived 3.9 billion years ago, we share more recent ancestors with other, more closely related organisms. For example, the ancestor of all living mammals probably lived in the early Jurassic period—around 200 million years ago. The last common ancestor of humans and chimpanzees (our closest living relative) probably lived sometime between 6 and 10 million years ago. The more closely related organisms are, the more recently their common ancestor lived. And, since a more recent common ancestor means less time for evolution to occur, more closely related organisms generally share more traits.
In sum, the fact that all organisms are unified by certain common traits—which revel their common ancestry—and at the same time show tremendous diversity in their morphology, physiology, ecology, and life history, among other things, is compelling evidence that evolution has occurred.