Is there a tree of life?

An alternative to the tree of life

An alternative to the tree of life

Ok, bear with me. The idea that there is a tree of life–a genealogical tree that links together all living things, past, present, and future–is often taken to be one of Darwin’s great contributions. While the idea pre-dates not only Darwin but pretty much all scientific attempts to think about how living forms are related to one another, the claim that there is a tree of life is often taken to go hand-in-hand with the idea of evolution, especially “by natural selection”, producing a pattern of species that looks very much like a tree over time, with branchings representing speciation events, and higher parts of the tree having many more branches than lower parts of the tree.

Over about the last 10 years, some little biologists–microbiologists–have called into question this picture of evolution. The push here has come largely from viral and bacterial systematists and those studying the “early history of life”–that is, life older than 1 billion years, aka the first 2/3 of the history of life. It’s not that anyone sane thinks that evolution has not taken place in the microbial world. Rather, folks have been wondering whether the pattern that evolution has produced much resembles a tree. The two chief phenomena that has driven scepticism here are lateral gene transfer (LGT), whereby genetic material is transferred across lineages, and endosymbiosis, where previously free-living organisms either insert themselves into (or are captured by, depending on your perspective) other, typically larger, free-living organisms. (Both chloroplasts in plants and mitochondria in eukaryotic cells more generally were gained by endosymbiosis.) Both kinds of phenomena are likely to be much more frequent in the microbial world than had been suspected as little as 10-15 years ago, and both raise questions about common ways of thinking about what organisms are, and how they are related to one another. If LGT and endosymbiosis are prevalent throughout the first 2/3 of the history of life, and they have left their indelible mark on all organisms now, then instead of a single tree of life, the history of life might well look more like the fractured, reticulated, cone-like structure pictured here.

There’s lots for philosophers of biology, and biologists themselves, to be interested in here. But why should anyone reading the What Sorts blog scratch this microbial itch? Three reasons, in brief.

First, there is an order of magnitude more microbial cells in the human body than human cells. Understanding their nature is perhaps key to understanding aspects of our own. At least someone like John Dupre, director of Egenis @ Exeter, thinks so. Second, with the rise of the new “germ theory of disease” in the hands of Paul Ewald and others, it’s not just acute diseases but many chronic diseases–arterioschlerosis, heart conditions, many cancers–that will likely be confirmed to have a microbial agent as their primary cause. To understand disease, we’ll need to pay much more attention to these living agents of destruction and understand their evolutionary trajectories, both short- and long-term. And, finally, at least for now, apart from its effect on our understanding of the very real world of human disease, coming to grips with LGT and endosymbiosis will likely have significant implications for how we think about related concepts, such as inheritance, family traits, and normal functioning. For example, biological inheritance need be neither genetic nor strictly vertical, if microbial agency is as much of a force in the history of life as the sceptics about the tree of life think. In fact, just how much biological inheritance is of the genetic, vertically-transmitted (parent-offspring) variety that is assumed to predominate, becomes an open question.

For more on the ToL, check out the recently formed Tree of Life Network, headed by Maureen O’Malley and based at Exeter, UK..

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