Life is messy.
We are taught from a young age that evolution happens in one direction. As we move forward in time, genes are passed from parents to children, like branches on a family tree. We, the current generation of life, can sit high in the branches and look downward, smiling, at the unbroken line to what must have been the first genesis of life. A clean shot from cell to animal to human.
Except it’s not that clean. What I’ve just described is known as vertical gene transfer and is, for most of us, the primary means of moving genetic material. However, there’s also ways to get around this, to exchange information between currently living organisms. This is known as horizontal gene transfer.
There’s a number of different ways this occurs. Bacteria can actually pick up and incorporate random exogenous DNA fragments (a process known as transformation) or conjugate and exchange DNA from cell to cell. Viruses can litter their hosts genetic code with leftover sequences from their original home. Cells can actually become trapped within other cells (a process known as endosymbiosis) and evidence suggests that DNA transfer occurs here too. Endosymbiosis is actually probably the reason your own human cells have mitochondria (and chloroplasts, if you’re a plant).
It’s quite likely that all of these were occurring at tremendous rates while life was first evolving. There was likely no first bacteria or eukaryote. Instead life truly was a ‘soup’, a fluid evolving community instead of distinct lineages. As Scientific American put it: “If there had never been any lateral gene transfer, all these individual gene trees would have the same topology (the same branching order), and the ancestral genes at the root of each tree would have all been present in the last universal common ancestor, a single ancient cell. But extensive transfer means that neither is the case: gene trees will differ (although many will have regions of similar topology) and there would never have been a single cell that could be called the last universal common ancestor.”