The tree of life is a metaphor used to describe the relationships between organisms, both living and extinct as described in Charles Darwin’s Origin of Species. The tree is still the main way that scientists can classify organisms and map out their evolutionary history.
When Charles Darwin first proposed his theory on the origin of the different species, he categorized the different life forms by their common characteristics, putting them in similar groups. Using this method, reptiles were distinguished from amphibians and amoebas from protists, single-celled animals that don’t fit into regular classifications.
With the new tree, it was necessary to redraw it to accommodate the literally thousands of new forms of bacteria discovered, making up almost two-thirds of the tree. It this isn’t enough to shake a few branches, the tree was not given a top. It sort of looks like a tumbleweed, says Bloomberg View. And some of its branches, like the ones leading to E. coli are as long as the branches leading to humans.
Genetic sequencing has allowed us to use genetic similarities to determine how every organism is related to another, but further genomic studies, using a specific gene, one that encodes a subunit ribosomal RNA, required to make protein was studied. But even these rRNA genes proved to be too similar, reports Ars Technica.
So to fish out the gene specific to an organism, the scientists ended up using the genomic date from all of an organism’s DNA to form the new tree. Dr. Jillian Banfield at UC Berkeley, one of the authors of the study, and her laboratory looked at thousands of genomes from the three traditional branches of the tree, Bacteria, Archaea, and Eukarya, to make the new tree.
They found there was a huge number of bacterial life forms that have never been cultivated in a laboratory setting. The bacteria are so tiny, being less than 0.1 microns in size, that their genomes are almost five times smaller than E. coli’s. These organisms lack the genes for even the basic metabolic processes and have been determined to be symbionts, requiring a partner in order to live.
Eukarya, the branch that includes all multicellular life, like honeybees, roses, whales and wolves, and of course, humans, actually make up a small fraction of the tree of life. The study says this makes sense because Eukarya haven’t been on Earth anywhere near as long as the other branches.
People today are often hazy on what they think the theory of evolution means. Michael Weisberg, a philosopher of science, says that people often believe that the tree of life depicts our progress up the tree from animal to human. That is not exactly correct, he says.
“As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever-branching and beautiful ramifications.” Charles Darwin, 1859.
What is amazing about the tree of life is that it is still growing. Evolving might be a better word to describe the tree. At the same time, extinctions are rapidly pruning the tree as species disappear forever. But according to the study, the scientists say our role as humans on the tree makes us very special.
While other animals can drive their prey or competitors to extinction, humans have that one unique ability that allows us to stop and think of the consequences. We are the only animal that has the power to choose which branch to prune.
The study, “A New View of the Tree of Life,” was published in the journal Nature Biology on April 11, 2016.
