Within biology, the tree of life is a graphic representation of the various lineages of all modern organisms. Each branch represents a large classification of living things, and each species sits at the tip of one specific twig. Whatever the distance between these tips, the various paths life has taken over the eons join together at the trunk, which may be seen as the ancestor of all life forms.
Humans occupy only a single twig in the outermost shell of the tree of life. Traveling back from that tip is effectively traveling back through time to a series of junctions with similar bands of related organisms. To travel 6 to 8 million years back along the human twig is to make the first rendezvous with the two species of chimpanzee, who had themselves joined halfway along their own path. Going further back up the branch, the human/chimpanzee group joins with gorillas, then Old-World monkeys, New-World monkeys and prosimians, such as the lemur family.
Eventually, the journey joins mammals to reptiles, vertebrates with all animals and, eventually, eukaryotes with prokaryotes. Each modern form, from cyanobacteria to elm trees and fungi, occupies a distinct point. These points are separated by, in the case of archaea and eukaryotes, perhaps billions of years, but every single living thing that has ever lived has its own path back up the branches to the common trunk.