The Microbial Eve: Our Oldest Ancestors Were Single-Celled Organisms
If Victorians were offended by Charles Darwin's claim that we descended from monkeys, imagine their surprise if they heard that our first ancestor was much more primitive than that, a mere single-celled creature, our microbial Eve.
We now know that all extant living creatures derive from a single common ancestor, called LUCA, the Last Universal Common Ancestor. It's hard to think of a more unifying view of life. All living creatures are linked to a single-celled creature, the root to the complex-branching tree of life. If we could play the movie of life backward, we would find this little fellow at the starting point, the sole actor in what would become a very dramatic story, lasting some 4 billion years.
There were, very possibly, other life forms before LUCA. We don't know exactly who LUCA was, or when it thrived. But paleo-biologists — scientists who investigate creatures that lived a long time ago — have succeeded brilliantly in mapping life's evolution from bottom-up in extraordinary detail, especially considering the difficulties in finding fossil evidence of creatures living billions of years ago. Instead of looking for bones or imprints in rocks, to find LUCA they look at DNA. They are able to trace LUCA to a simple prokaryotic creature (a single-celled bacterium with unprotected genetic material) that lived some 3 billion years ago. It must have been a very tough organism, able to survive in very extreme environments.
The tree of life is pretty complicated. However, if you look at the picture, you will learn two important things: first, that humans and other animals are the absolute minority, a twig at the bottom right as part of the eukaryotes, organisms with cells that have DNA as genetic material protected by a membrane. (Eukaryotes include animals, plants, fungi and protozoans.) Second, that the vast majority of living creatures are bacteria.
Next to the eukaryotes you will find the archaea, also single-celled organisms that are able to survive in extreme environments, such as near hot underwater thermal vents or oxygen-free wetlands. All evidence indicates that LUCA was a primitive form of archaea.
Evolutionary biologist William Martin, from Heinrich Heine University in Duesseldorf, Germany, tried to track down LUCA in the genes of bacteria and archaea. This is not an easy task, as organisms often swap genes, making it hard to know what came from a very ancient lineage and what was picked up more recently.
Martin's strategy was to search for genes found in at least two kinds of modern bacteria and archaea; this would indicate that the gene has been inherited from distant ancestors, as opposed to being a random recent pickup.
After analyzing genes from 2,000 modern microbes sequenced over the past 20 years, the researchers found 355 gene families that appeared frequently among the microbes, suggesting that they shared a common origin. Once analyzed, the DNA evidence indicated that LUCA was anaerobic (lived in the absence of oxygen) and thermophilic — that is, heat-loving. As Martin and collaborators wrote:
"LUCA inhabited a geochemically active environment rich in H2 (hydrogen gas), carbon dioxide and iron. The data support the theory of an autotrophic [organisms able to feed from simple inorganic substances] origin of life...in a hydrothermal setting."
In other words, according to these results, LUCA was likely a simple one-celled organism that lived where seawater and magma met at the ocean floor, the so-called hydrothermal vents.
There are, of course, critics of the theory, who argue that life originated instead on land and migrated to underwater habitats to protect itself from difficult conditions on the surface — due to intense and frequent meteoritic impacts that died down around 3.9 billion years ago. The answer, if it can be found, will depend on whether there are indeed any extant biochemical signatures of such primitive terrestrial life, a difficult task due to the constant churning of ancient rocks.
For the moment, evidence points to our microbial Eve as a tough underwater organism, able to thrive in very hard conditions. We should expect this from any organism that branched out to become every other creature that ever lived. Talk about genetic legacy!
Marcelo Gleiser is a theoretical physicist and writer — and a professor of natural philosophy, physics and astronomy at Dartmouth College. He is the director of the Institute for Cross-Disciplinary Engagement at Dartmouth, co-founder of 13.7 and an active promoter of science to the general public. His latest book is The Simple Beauty of the Unexpected: A Natural Philosopher's Quest for Trout and the Meaning of Everything. You can keep up with Marcelo on Facebook and Twitter: @mgleiser
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