A tree of life showing the common ancestry of life on earth is one of the most powerful metaphors in science, one which stood the test of time remarkably well. The concept of a taxonomic tree as a means to arrange species can be attributed to the German naturalist Pallas in 1766. At his time, species were ordered in a ladder-like fashion (Scala Naturae) where ‘less perfect’ organisms were put at the bottom of the ladder. Climbing the ladder, you eventually came across humans and in the end, God himself. Attacking this idea, Pallas says:
But on the contrary, let the System of all organic Bodies be well represented by a the likeness of a Tree, which at once from the base, out of the simplest plants and animals, twofold, it brings forth diversely a contiguous trunk, Animal and Vegetable.
Darwin's sketch of a Tree of Life, as it appeared in 1837 (Wikipedia)
So using the tree as a metaphor for classifying nature is an old trick. The French Biologist Lamarck was close to publishing a true evolutionary tree in his Philosophie Zoologique in 1809 (the year Darwin was born), you can find his figure here. In this figure, you can see that Lamarck correctly assumes that birds evolved from reptiles. Incorrectly, he thought that lineages of species followed separate paths in evolution (for example, buffaloes and whales both evolved from reptiles, but from different species of reptiles, taking different paths) on their way to perfection. It was Darwin himself who unified the taxonomic tree with the concept of common descent: every species is related to each other species, and their last common ancestor can be found at the branchpoint of the branches that will lead to the respective species.
In theory, the tree of life encompasses and shows the relatedness of every species that lives or has lived on earth. Every branch and twig leads up to the different species that are alive today: from chickens, to yeast, to whales. Chronicling life’s history: an impressive feat for a metaphor that is so easy to grasp! As an aside, if such a tree were to exist in reality, the majority of its branches would be dead, since 99,9% of the species that ever lived on this planet became extinct at one point in time. Such a tree would be a sorry sight indeed and in shrill contrast to the vibrant artist renderings of the tree of life!
But alas, the tree of life as an analogy for life’s history has its limits. While it works perfectly well for us species that are clearly separated from one generation to the next (ask your grandmother!), it seems to fail as a model for the majority of living species on this planet; microbes and their ilk. These so called prokaryotic species have more than one way to transfer their genes. We as humans only transfer our genes from generation to the generation; but prokaryotes can also transfer their genes ‘horizontally’, which has been dubbed horizontal gene transfer (HGT). This means that two microbes can exchange their genetic material, without them being related to each other in any way. This complicates the picture, since the flow of genes no longer coincides with the flow of species over time. In the figure below, the interflow of genes between prokaryotic species is visualized. There is no single unbroken line from the earliest forms of life to the bacteria that live today: a network or web connects them. This is closer to the truth than you might think: in a recent study, scientists found that on average, 81% of genes in the genome of a prokaryotic species underwent HGT at some point in history. To the right, animals, fungi and plants branch off, hinting at the tree of life we all know and love.
A modern take on the Tree of Life (Uprooting the Tree of Life)
If genes can be shared so easily between one species to the next, can you still consider prokaryotes members of different species, or is it more appropriate to talk of a shared gene pool, which manifests itself differently over time? What does this mean for the evolution of current and early prokaryotic life? These are some of the questions scientists are facing now. It’s beautiful to see how far the concept of the tree of life has taken us so far. I’m deeply curious as to where it will take us, as science delves deeper into life’s history.
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