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What Was The Last Universal Common Ancestor (LUCA)
"Therefore I should infer from analogy that probably all the organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed."
- Charles Darwin, On The Origin of Species
The Theory of Evolution predicts that all life descended from a single common ancestor. In the metaphor of the Tree of Life, this organism represents the trunk from which emerged the three branches of all life on Earth. This ancestor was not necessarily the first life to emerge on Earth, merely the one from which all others began to branch. Scientists studying the origins of life refer to this hypothetical organism as LUCA, or the Last Universal Common Ancestor.
Biologists currently classify all life into three main branches: eukaryota, bacteria, and archaea. The eukaryota branch contains all plants, animals, protozoa, and anything with a cell nucleus. The bacteria and archaea branches contain single-celled organisms without cell nuclei, and had previously been grouped together under the term prokaryotes. However, more recent research into archaea has revealed them to have a significantly different biochemistry than bacteria, leading to their taxonomic reorganization into an entirely separate branch of the tree.
Determining what sort of organism comprised the trunk of this tree would seem to be a fairly simple procedure. Biologists should be able to compare the genetic and physical characteristics of the members of all three branches, determine which components they all have in common, and declare that the Last Universal Common Ancestor was an organism with all of these shared characteristics.
Unfortunately, nature has a habit of defying the models we create to explain it, and thus the search for LUCA is considerably more complex than simply compiling all of the shared genes.
The Tree of Life
What We're Pretty Sure We Know About LUCA
Although there is some considerable uncertainty and disagreement among paleobiologists about what the Last Universal Common Ancestor was and what characteristics it had, there are a few points on which most can agree:
- LUCA lived during the Paleoarchaean era, 3.5-3.8 billion years ago;
- LUCA was a single-celled organism with a lipid-based cell membrane;
- LUCA had a nucleotide-based genetic code (though whether this was DNA or RNA is still in dispute);
- LUCA's genetic code used three base-pair codons to code for amino acids;
- LUCA had ribosomes that assembled amino acids into proteins;
- LUCA was able to replicate itself by division.
This may seem like a rather obvious list of commonalities between all life on Earth, but it contains some important implications. The mere fact that we do not see, say, four-nucleotide codons or other forms of alternate biochemistry in existing species strongly imply a single ancestry for life, and thus a universal ancestor.
Figuring out exactly which genes this ancestor had is no easy task, however. And it is a task made more difficult by the communal habits of many forms of life.
More Origin of Life Reading
How many common ancestors do you think there were?
Gene Swapping and Communal Cells
Horizontal gene transfer, more colloquially referred to as gene swapping, makes life difficult for researchers trying to determine which genes in the genomes of all living things date back to LUCA.
The trading of genes is a common practice in bacteria today, and it is one of the ways in which a population of bacteria can quickly build up antibiotic resistance. Paleobiologist Carl Woese believes that this was a widespread practice among the earliest forms of proto-life, allowing the emerging life form(s) of the paleoarchaean era to adjust to changing environments.
Horizontal gene transfer also occurs between the major branches of the tree of life. The mitochodria in our cells was once an independent form of bacterium, and carries its own DNA. There is some evidence suggesting that some genes from this mitochondrial DNA have been swapped into the chromosomes of our cell nuclei. Other studies have found a significant portion of the human genome to have derived from bacteria and viruses.
All of this gene swapping makes reconstruction of life's family tree difficult. It can be difficult to tell whether a universal gene has been present since the beginning of life's evolutionary branching, or is a more recent addition swapped in to the main branches over time.
Just How Many LUCAs Were There?
The difficulty in establishing what the genome of early life was is just part of the puzzle created by gene swapping. This has also caused some paleobiologists to question whether there even was a single common ancestor, or if life arose independently from multiple ancestors that simply swapped enough genes to give the appearance of common ancestry. Specifically, some commonalities in biochemistry between archaeans and eukaryotes have led several biologists to suggest that bacteria does not share an ancestor with them, but is part of a different tree of life altogether.
This idea was put to the test in a 2010 paper by Douglas L. Theobald of Brandeis University's Department of Biochemisty. Theobald analyzed both of these models of life's origin using model selection theory, a method of statistical analysis used to determine which of a series of models best describes observed results. Using the genomes of four species each from the eukaryote, bacteria, and archaean kingdoms, Theobald ran thousands of simulations showing how these differing genomes would have arisen, including the possibility of horizontal gene transfer.
Theobald's results were overwhelmingly in favor of a single origin for all life, rather than multiple origins. "UCA is at least 102,860 times more probable than the closest competing hypothesis," wrote Theobald in his analysis of the study's findings.
The Elusive LUCA
While the findings of this and many other studies have offered some insights into what the common ancestor of all life may have been, it is possible that this question may never truly be answered. The search for clues to the Last Universal Common Ancestor's identity will likely continue indefinitely, with new findings only adding to the mystery of the elusive ancestor of all life on our planet.
Sources and Further Information
- What is the Last Universal Common Ancestor (LUCA)?
Anthony M. Poole. In the study of early life on Earth, one name towers above the rest: LUCA.
- Mapping the Tree of Life: Progress and Prospects
Norman R. Pace. The intent of this article is to provide a critical assessment of our current understanding of life's phylogenetic diversity.
- Interpreting the universal phylogenetic tree
Carl R. Woese. The universal phylogenetic tree not only spans all extant life, but its root and earliest branchings represent stages in the evolutionary process before modern cell types had come into being.
- Collective evolution and the genetic code
Kalin Vetsigian, Carl Woese, Nigel Goldenfeld. The central concept is that a variety of collective, but non-Darwinian, mechanisms likely to be present in early communal life generically lead to refinement and selection of innovation-sharing protocols
- On the origin of genomes and cells within inorganic compartments
Eugene V. Koonin and William Martin. We suggest that, within a hydrothermally formed system of contiguous iron-sulfide (FeS) compartments, populations of virus-like RNA molecules ... became the agents of both variation and selection.
- A formal test of the theory of universal common ancestry
Douglas L. Theobald. The classic evidence for UCA, although massive, is largely restricted to ‘local’ common ancestry— for example, of specific phyla rather than the entirety of life—and has yet to fully integrate the recent advances...