One of the most fundamentally important unanswered questions for science is the origin of life.
We have a good understanding of the biochemical mechanisms involved in living cells and have been able to generate cellular creatures using minimal DNA. An artificial bacteria employing 473 genes has been generated by scientists at the J. Craig Venter Institute, which is comparable to the 525 genes found in Mycoplasma genitalium. But having generated a bacteria, how do we get to more sophisticated or complex organisms?
Life, as we know it, has three major stems: bacteria, archaea and eukaryotes. Bacteria and archaea are sometimes lumped together as prokaryotes as they have no nuclei, lysosomes, mitochondria or skeletons. They are the most ancient of all organisms and the most plentiful. Indeed, if an alien race visited our planet in search of the dominant life form, bacteria and archaea would win hands down.
Two billion years ago, something happened. Complex cells emerged as eukaryotes arrived on the scene. All complex life - which includes pretty much anything living you can see - derive from a common ancestor. We know this because all of these cells share common genetics, produce specific proteins and, in many instances, function in the same way. Mutations have happened in complex multicellular structures but there is a surprisingly large number of genes which can be found in every single organism.
How did life shift from single-celled bacteria and archaea to eukaryotes? There are a number of hypotheses but work on one particular class of organisms may yield some answers. A new species, called Prometheoarcheum syntrophicum, appears to be a transitional form between the two stems, as reported in Nature.
While our cells are stuffed with a multitude of containers such the nucleus which holds DNA and the fuel cells called mitochondria, Prometheoarcheum lacks these structures but still contain the genetic code for the proteins which are utilized for their construction. It is a bit like finding a pile of wood, shingles, wallboard, nails, and such only to discover they are not being used to build a house but to construct a car. The essential eukaryotic proteins are present but not being used in the way we would expect.
In 2015, Thijs Ettema of Uppsala University in Sweden and his colleagues were able to scoop DNA fragments from Arctic Ocean sediments which appeared to be transitional between a species of archaea and eukaryotes. Ettema named their source Asgard archaea as Asgard was the traditional home of the Norse Gods. This same DNA was subsequently discovered in a river in North Carolina, a hot spring in New Zealand, and various other places but the organisms themselves remained elusive.
In 2006, Masaru K. Nobu, a microbiologist at the National Institute of Advanced Industrial Science and Technology in Japan, and his colleagues collected sediment samples for the Pacific Ocean. They were attempting to isolate microbes capable of eating methane, which would be useful for cleaning up landfills.
They kept their samples in a methane-rich environment at the temperature and pressures found on the sea floor. The mud contained many different microbes but by 2015 the researchers had isolated a particularly interesting new species of archaea which turned out to be one of the Asgard archaea discovered by Ettema. The research team then undertook the arduous task of cultivating and characterizing the new species. They have now been able to ascertain its genetic heritage.
In total, it took 12 years to get to the point where they now have a stable species which can be observed with a microscope. And under the microscope, it is a strange beast. The microbe starts life as a tiny sphere but sprouts long branching tentacles and releases methane-covered bubbles as it matures. And even more surprising is the lack of the internal structures expected based on its genetic make-up. It appears to truly be a transitional step between the two domains.
Before the discovery of Prometheoarcheum, many researchers suspected the missing link was a predator which consumed smaller microbes to supply its internal organ structure. For example, we know mitochondria have their own DNA and are essentially symbiotic organisms found in every one of our cells. But with Prometheoarcheum, there is some evidence to suggest a slow assimilation of other microbes rather than a wholesale takeover. As a consequence, Nobu's team have proposed a new mechanism in which cells live in clusters, eating each other's waste and supplying needed nutrients before the eventual merger. In modern parlance, they live together before finally getting married.
We still don't have all of the pieces in the long complex struggle from chemical compounds to modern elephants but some of the links are becoming clearer. Continued research on Promethearcheum may finally allow us to bridge the gap from prokaryotes to eukaryotes. Eventually we might be able to answer the question of how life began.
