When biologists think about life on earth, they think trees.
Not the trees shading us on clear sunny days, but metaphoric trees that depict the evolution of life. The premise is simple. The tree begins with a node representing the ancestor of all current life. Two branches extend from that node in opposite directions connecting to two new nodes representing two new ancestors from which more branches extend. The pattern continues many millions of times until there aren’t any more ancestors, leaving a series of branches at the end of the tree.
Those branches represent today’s life. More specifically, they represent the species that exist on the planet.
The tree of life is a basic model of how evolution happens. New species form when a common ancestor splits into two new species through a process called speciation. Once the two species are formed, they accumulate differences over many millions of years and go through cascading splits that continuously create the biological diversity of species we see on earth today. But what happens when the real world has so many exceptions that the tree of life metaphor can’t hold up to reality?
This is where an alternative metaphor becomes useful: the coral of life.
In this version, nodes can connect with each other from distant parts of the biological world, species can be formed through many connections (not just one), and life suddenly gets a whole lot more complicated. So why even use this metaphor when the tree of life has already taken hold of our hearts and minds?
To understand the reasons for changing our metaphorical perception of how life works, we have to understand how evolution happens. Living things evolve into new living things when their genetic architecture changes. Differences in this architecture arise when a population of a species is separated into different populations because of some non-biological force, i.e. a mountain range. Once these differences are created, it all depends on which individuals are best able to survive and reproduce into a new generation in whatever environment they end up in (natural selection). Those individuals will carry on their differences into the future, eventually changing the composition of their population and, consequently, the species. Thus, speciation has occurred.
This is the most basic model of evolution, but as science progresses, we discover that life is a lot more complex. The first example happened over a billion years ago when one cell swallowed another.
Instead of accumulating differences through time and gradually becoming different, our cells evolved through a fateful relationship formed when two branches of the tree of life connected to one node. A large cell incorporated a different cell into itself. Over time, the swallowed cell became what we see today as a mitochondrion, the energy powerhouse that animates us. Without mitochondria, we would not be able to create the energy we need to think, move, breath or grow, among all other things.
Even today, different strains of bacteria swap genetic material back and forth, creating antibiotic resistance. Viruses inject new genetic material in the living things they infect. There are even molecular parasites in the form of small segments of DNA that cut-and-paste themselves into foreign DNA and propagate throughout the population of a species.
The more we look deeper into the microscopic world underlying the life we see all around us, the more we realize that a tree of life may be unrealistic. It’s missing key information still mysterious to science.
Unlike real trees, our metaphoric tree of life is only alive at its tips, where today’s species exist. Corals, on the other hand, build themselves up from the non-living skeleton they form beneath.
Diler Haji is a staff writer for The Daily Campus and can be reached via email at firstname.lastname@example.org.