An Incredibly Complete Tree of (Part of) Life

This remarkable image greeted me when I opened my Twitter feed yesterday morning:



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Researchers gathered nearly 7,000 evolutionary trees from 3,000 studies and fed them into a database. They then combined 500 of those trees with taxonomic data from the Open Tree Taxonomy (to help with groups where an evolutionary wasn't available) to make this megatree. It's the first attempt at a comprehensive tree of life, showing the relationships between all the 2.3 million types of creatures we know about. The team has made the tree and the database behind it available online so we can browse through it and other researchers can add their data. It's an impressive bit of work and an astonishing picture. It's also humbling, not because of what it shows, but because of what's missing.

The tree may be comprehensive, but it's not complete. Far from it. "As important as showing what we do know about relationships, this first tree of life is also important in revealing what we don't know," said author Douglas Soltis in a press release. There are lots of creatures we know very little about, and, obviously, they don't show up in this tree. This seemingly trivial fact matters because our ignorance is not evenly distributed. In another figure, the researchers compared the number of species estimated for each group with the number they could include in the tree. Animals (that's the "Metazoa" that make up the top half of the circle") are pretty well accounted for, though only about 1 in 10 estimated insect species and 1 in 100 nematode species are included. Likewise, there should be roughly 10 times for species of fungi. That might seem like a lot of species, but the overwhelming absence — the huge elephant that's not in the room — is bacteria. This tree may be depicting as few as 1 in 10,000,000 types of bacteria. In that case, the 'bacteria' slice (currently from 3-4 o'clock on the cirlce) would cover most of circumference.

Of course, we don't really know how many kinds of bacteria there are. The issue is even less clear because it's hard to agree on what 'species' means when talking about bacteria. At the lower bound, there may only be 100 times more bacterial 'species' than are in the comprehensive tree — a more modest distortion. Whatever the actual figure, my point is that this tree is a lens into our own bias. It's a remarkable and praise-worthy accomplishment, but, at the same time, it doesn't show the richness and diversity of the biological world but of the parts we've chosen to look at.

Ref
Hinchliff, CE, Smith, SA, Allman, JF, et al. Synthesis of phylogeny and taxonomy into a comprehensive tree of life. PNAS Early Edition. (2015) doi:/10.1073/pnas.1423041112

Image credits
The tree of life is a figure in the paper.