Lessons from the Tree of Life

The Tree of Life tells us that all living creatures are related; that for billions of years, all  life on Earth was microbial; that cooperation is essential for evolution; that horizontal gene transfer explains much of the diversity we see; and that, through extinction, some branches get cut down.

For an excellent review,  see: Evolution of genes and organisms: the tree/web of life in light of horizontal gene transfer, by Lorraine Olendzenski and J Peter Gogarten.

Web of Life

The Tree of Life is actually more like a rooted “Web of Life.”  Growing DNA sequence data shows that organisms have evolved via horizontal and vertical gene flow.  Because of horizontal DNA transfer, genes and genomes are passed  horizontally as well as vertically.

For more detailed information, check out:

Beyond the tree of life, Edited by: Dr Maureen O’Malley and Dr Yan Boucher, Collection published: 30 June 2011.

A Rooted Net of Life by David Williams, Gregory P Fournier, Pascal Lapierre, Kristen S Swithers , Anna G Green, Cheryl P Andam and J Peter Gogarten.

The Net of Life: Reconstructing the microbial phylogenetic network. Victor Kunin, Leon Goldovsky, Nikos Darzentas, and Christos A. Ouzounis

We are All Related

Monarch butterflies, monkeys and microbes are all related. The last universal common ancestor of life (LUCA) is indicated at the bottom of this image.

Life Over Time

When did life emerge?  When did the first nucleated cells form?  These events happened so long ago, that we only have rough estimates about times. The numbers on the left indicate the billions of years ago that events occurred (approximately). At this scale, humans emerged in less than a pixel distance from the present. Dinosaurs went extinct several more pixels back.

Mobile DNA via Natural Genetic Engineering Drives Evolution

The white horizontal lines of this tree indicate horizontal gene transfer.  We now know that among the prokaryotes, mobile DNA transfers easily from one organism to another.    The most dramatic kind of horizontal gene transfer is the symbiosis of two different organisms to create an entirely new organism.  For example, the convergence of a bacterium (seen as blue) with a eukaryote gave rise to mitochondria, the energy powerhouses of our cells. Another symbiosis of a green, photosynthesizing cell with a eukaryote gave rise to all the plants. Horizontal gene transfer allows for the sharing of mobile DNA, which is a powerful driver of evolution.

In the first billions of years of life on Earth, and for prokaryotes today, the tree of life is more like a network than a tree. Genes passed freely from one species to another. Today, Bacteria and Archaea continue to swap their genes between completely different “species”.   In eukaryotes, DNA mobility is facilitated by genetic recombination during meiosis, which in turn drives rapid evolution. Although gene swapping between species is rare in the eukaryotes, this does occasionally happen, especially via mobile DNA carried in viruses.

Pruning the Tree

Lines that end before the top of the tree depict branches in the tree of life that have been pruned by extinction. More species have gone extinct than exist today. The good news for us as humans, is that we are among the survivors! Furthermore, we are the first creatures to evolve that have the foresight that may well help us prevent calamities, such as the meteor strike that wiped out the dinosaurs.

DNA Is the Most Reliable Evidence

This tree is based on comparisons of DNA sequences from genes of thousands of diverse creatures. Humans, amoebas, strawberries and E. coli all share several hundred similar genes. (For comparison, humans have between 20,000-25,000 genes.)

Origin of Life is Speculative

The dotted lines at the base of this tree indicate our lack of certainty as to how the first life began.   Surprising evidence suggests that this ancestor may actually be not a single organism, but rather a group of organisms that freely exchanged their DNA via horizontal gene transfer, as indicated by the white lines at the base of the tree.

All Models Are Wrong, Some Models Are Useful

This tree of life is vastly simplified for the sake of a presenting a clear overview. It provides a useful summary of the relationships of all creatures to one another and roughly when they appeared in time. Like all models, this image has several limitations. Obvious examples are that this images does not show the actual numbers of living creatures and it depicts only a limited number of instances of interspecies gene transfer. As more information becomes available, the branches of this tree will surely change. The origin of life is purely speculative. It may have occured once or several times. This tree also is a very rough approximation as to when various events occured. Despite these limitations, this tree does give viewers a fuzzy snapshot of how life on Earth evolved.

Understanding Through Classification

To help us understand the relationships of all living things, scientists classify organisms into groups.  There are many different classification schemes, just as there are different maps of the same that help people navigate roads or see major scenic spots.   This image shows only one of several classification schemes, the Three Domain system of Carl Woese.  The blue Bacteria and red Archaea are “domains” of organisms that lack a nucleus. The plants, animals, fungi, and protists are all members of  the domain of “Eukarya”, those organisms with a defined nucleus.