Some 3 billion years ago, life on Earth is presumed to have settled down to a single source from which all life presently on the planet descended. This last universal common ancestor (known as LUCA for short) is not presumed to be the first that arose on Earth, but the last one from which every living thing today evolved.
Now, a strange picture of this life is starting to emerge from the work being done to investigate it.It seems to have ben vast – a single super-organism inhabiting all the oceans of the whole world. By the time LUCA was living in the seas, it seems to have been a cellular form developed through perhaps a billion years of evolution and genetic swapping that turned the entire ocean into a vast genetic swamp of primitive life, swapping genetic material more or less freely. Then, about 2.9 billion years ago, it deeloped into the first three domains 0f life – the archaea, the bacteria nd the more complex eukaryotes.
It is almost impossible to know what happened before that split since no fossil remains exist from that time – and, indeed, precious few from later on as well. Even without though, we can look into the nature of LUCA thanks to the structure of the proteins it is likely to have used. If we find, for instance, a protein that every organism alive today uses then we can be fairly sure that the same protein was also in use within LUCA or at leat that there was a protein with the same overall structure.
While it is possible that similar structures have evolved in different domains, it is probable that such common structures represent a form of living fossil that can give us a view on the capabilities that were common throughout the mega-organism we know as LUCA. Caetano-Anlles searched for such bio-fossils through a list of 420 modern organisms and the proteins they produce and found just 5 to 11 percent were universal meaning that they were sufficiently conserved to be likely constituents of LUCA.
So what could LUCA do? Examining the function of the conserved structures, it seems it could use enzymes to extract energy from nutrients and had some primitive protein making equipment but that it lacked the wherewithal to make or read DNA molecules. If it came in the form of cells, then LUCA must have been able to make membranes and Armen Mulkidjanian from the University of Onasbruck has traced the history of membrane proteins to find out what sort. He concludes that LUCA could oly make very simple isoprenoid membranes, which are very leaky compared to modern membranes.
Primitive it may be, with a leaky membrane, probably no organelles (although there are suggestions of a primitive sort of acidocalsome) and using RNA rather than DNA, but we are now begining to narrow the field and identify what our progenitor life was like. Its genetic structure could not be well used to build proteins because of the high error-rate, but it worked, and its membrane was a leaky mess, but also worked. Setp by slow step, we are coming to pin down more and more of the history of life on Earth.
- An Earlier Genetic Molecule May Predate DNA And RNA (medicalnewstoday.com)
- The ancestor of all life on Earth might have been a gigantic planetary super-organism (io9.com)
- Researchers Show How Cellular Complexity Can Evolve (science.slashdot.org)
- Resurrecting extinct proteins shows how a machine evolves (blogs.nature.com)
- How did molecular machines evolve? Researchers resurrect lost proteins to find out (arstechnica.com)
- Enough About Intelligent Design (slog.thestranger.com)