Photo credit: Jill Banfield/UC Berkeley, Laura Hug/University of Waterloo
By Carl Zimmer
A team of scientists unveiled a new tree of life on Monday, a diagram outlining the evolution of all living things. The researchers found that bacteria make up most of life’s branches. And they found that much of that diversity has been waiting in plain sight to be discovered, dwelling in river mud and meadow soils.
“It is a momentous discovery — an entire continent of life-forms,” said Eugene V. Koonin of the National Center for Biotechnology Information, who was not involved in the study.
The study was published in the journal Nature Microbiology.
In his 1859 book “On the Origin of Species,” Charles Darwin envisioned evolution like a branching tree. The “great Tree of Life,” he said, “fills with its dead and broken branches the crust of the earth, and covers the surface with its ever branching and beautiful ramifications.”
Ever since, biologists have sought to draw the tree of life. The invention of DNA sequencing revolutionized that project, because scientists could find the relationship among species encoded in their genes.
In the 1970s, Carl Woese of the University of Illinois and his colleagues published the first “universal tree of life” based on this approach. They presented the tree as three great trunks.
Our own trunk, known as eukaryotes, includes animals, plants, fungi and protozoans. A second trunk included many familiar bacteria like Escherichia coli.
The third trunk that Woese and his colleagues identified included little-known microbes that live in extreme places like hot springs and oxygen-free wetlands. Woese and his colleagues called this third trunk Archaea.
Scientists who wanted to add new species to this tree of life have faced a daunting challenge: They do not know how to grow the vast majority of single-celled organisms in their laboratories.
A number of researchers have developed a way to get around that. They simply pull pieces of DNA out of the environment and piece them together.
In recent years, Jillian F. Banfield of the University of California, Berkeley and her colleagues have been gathering DNA from many environments, like California meadows and deep sea vents. They have been assembling the genomes of hundreds of new microbial species.
The scientists were so busy reconstructing the new genomes that they did not know how these species might fit on the tree of life. “We never really put the whole thing together,” Dr. Banfield said.
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