Naturalists once put living things on a scale of progress from primitive to advanced, with man as the pinnacle of creation, inspired by the Scala Naturae or Great Chain of Being conceived by Aristotle in about 350BCE. That ‘ladder of life’ was later replaced by the idea of putting all species – past and present – on one tree to represent evolutionary history. But does that ‘tree of life’ accurately reflect the relationships between everything that has ever lived?
Where did the metaphor come from?
Charles Darwin published On the Origin of Species in 1859. His book contains a single abstract diagram to illustrate ‘descent with modification’ (evolution), a V-shaped tree with groups of organisms as branches, leading to species as twigs. In 1866, Ernst Haeckel drew the first annotated ‘tree of life’, with three major branches: animals, plants and ‘protists’. The third group originally included anything that wasn’t fauna or flora, including microscopic organisms.
How are species put on branches?
Similar species are grouped together based on shared characteristics that are assumed to have existed further back along their branch, in a common ancestor. While the characteristics can be physical features such as anatomy – the only option for extinct species that have left fossils behind – living things are now typically grouped by genetic similarity. The practice of putting species into groups or clades (from klados, Greek for branch) is called cladistics and reconstructing evolutionary trees is phylogenetics (meaning origin of races).
Has the tree of life changed over time?
Yes. Old textbooks split life into five kingdoms: animals, plants, fungi and protists, plus ‘monera’ – single-celled organisms without a nucleus, now known as prokaryotes. But that changed in about 1977, after microbiologist Carl Woese discovered differences in the gene for producing 16S rRNA (a part of a cell’s protein-making machinery) among prokaryotes, suggesting they actually consist of two groups – namely bacteria and archaea – which are as distinct from one another as they are from the other kingdoms combined. Along with eukaryotes, whose cells have a nucleus, that gave three major branches on the tree of life.
Is the metaphor right?
Not entirely, as trees capture only a partial picture of evolution. A tree can only show how genes and their associated characteristics are inherited along a vertical route via a branch – how they’re passed down from one generation to the next (‘up’ the tree), from parent to offspring, ancestors to descendants. But genetic material can be transferred between two species on separate branches too. Such ‘horizontal gene transfer’ is rare in multicellular eukaryotes because a foreign gene would need to overcome two barriers before it could be inherited – entering a reproductive cell (sperm or egg) and then crossing into the nucleus.
Why does gene transfer matter?
It matters because horizontal transfer can offer new abilities that enable a species to adapt to its environment, such as providing superbugs with genes that confer antibiotic resistance. While rare in complex life, swapping genes is common among microbes. A comparison of distantly-related groups revealed that, on average, 40 per cent of a microbe’s genome (its complete set of genes) comes from ancient transfers. If you draw those gene transfers as connections between branches on the tree of life, the tidy structure ends up looking like a messy web or ‘network of life’.
So was Darwin wrong about the tree?
No, but its structure is hazy. The tree of life is really a ‘tree of cells’ whose ever-growing branches divide to reproduce. Like multicellular life, single-celled microbes also reproduce by dividing so, despite horizontal transfer, it should be possible to detect an underlying pattern of branches. In fact, scientists have studied networks of genomes and found ‘genetic worlds’ of eukaryotes, archaea, bacteria and viruses – which are connected by thin webs of gene transfer yet remain broadly discrete. If you draw those ‘worlds’ as branches of life, you see a backbone of evolution that (if you squint) still looks roughly tree-shaped.
Main image: U.S. Geological Survey/creative commons