Although the tools of modern molecular biology weren't available to early evolutionary thinkers, we now have at our disposal even more evidence that supports evolution. For starters, we know that similar chemistry, structures and processes underlie all cells. Consider, for example, that all cells, regardless of what animal they come from, have mitochondria. This important cellular organelle is the site for cellular respiration in almost all eukaryotic organisms. Ribosomes are another great example—cells from all three domains of life (Eukarya, Bacteria, Archaea) possess these structures, which help to assemble proteins based on the organism's genetic code. Ribosomes from cells of these three domains actually do show differences, but nonetheless, the process of protein synthesis basically works the same way among all living things.
Molecular biology also contributes to our understanding of evolution by revealing degrees of relatedness between different organisms. We just noted that some structures and processes are conserved across all organisms, providing evidence for shared ancestry a long time ago. But modern genetics can also help us parcel out more recent evolutionary events. By figuring out the sequences of certain genes and comparing them in different organisms, scientists can figure out who is most closely related to whom. This is because mutations occur every time our genetic material is replicated; it doesn't happen very often, but every once in a while a base pair might be added, deleted, or changed. It is estimated that 300 new mutations are introduced into the human genome every generation—that is, you have 300 new mutations compared to your mom and dad! Assuming these mutations occur at a somewhat constant, predictable rate, we can reason that the more time that has passed since two organisms diverged from each other, the more different their DNA will be. Just think, taking into account 300 new mutations every generation, your child would have 600 new mutations compared to your parents and your grandchild would have 900 new mutations. Over time, these accumulate in the genome, adding to the differences observed between organisms. In other words, more similar DNA sequences can mean that two organisms are closely related, and very different sequences indicate that they split from each other a longer time ago.
For aficionados of gigantic Ice Age mammals, mammoths provide a cool example of how gene sequencing reveals information about the evolution of elephants and their relatives. In 2006 an international group of scientists sequenced genes from extinct wooly mammoths—itself a remarkable feat. Mammoths are often found in permafrost, extremely cold soil, which provides ideal conditions for preserving DNA. The research team compared mammoth gene sequences to two different kinds of elephants living today: modern Asian and African elephants. They discovered that the closest living relatives of mammoths are Asian elephants. In other words, mammoths and Asian elephants share more of their DNA and a more recent common ancestor than modern Asian and African elephants.
Why did the Woolly Mammoth cross the road? Because they didn't have chickens in the Ice Age.