What Was The Last Universal Common Ancestor (LUCA)

Two-billion year old stromatolite fossils from the Mary Ellen iron mine, near Biwabik, Minnesota.
Two-billion year old stromatolite fossils from the Mary Ellen iron mine, near Biwabik, Minnesota. | Source

"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

Mapping of the Tree of Life based on sequenced genomes. Eukaryotes are pink, archaea are green, and bacteria are blue.
Mapping of the Tree of Life based on sequenced genomes. Eukaryotes are pink, archaea are green, and bacteria are blue. | Source

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.

A natural community of bacteria growing on a single grain of sand.
A natural community of bacteria growing on a single grain of sand. | Source

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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.

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Comments 16 comments

davenmidtown profile image

davenmidtown 4 years ago from Sacramento, California

Another awesome and detailed piece of work! LUCA seems like a far off thought...but one that nags at your conscious...


DzyMsLizzy profile image

DzyMsLizzy 4 years ago from Oakley, CA

In point of fact, if there WAS such a thing as a "LUCA," then the inescapable conclusion is that the entire human race descended from a combination of bestality and incest! So, there would actually have had to have been TWO such individuals, not just one...OR, we would not event exist, because a single individual multi-celled organism is generally incapable of reproduction. Only amoebas and other such lower life-forms can split and reproduce alone.

I think it is more likely that we were seeded here by an alien race. ;-)


buckleupdorothy profile image

buckleupdorothy 4 years ago from Istanbul, Turkey

Very very interesting, although I am eager to hear the rest of this sentence: "The transfer of genes from one organism to another..."!

Also, HAH re: DzyMsLizzy's theory that we were seeded by an alien race. Would love to see those scientists' notebooks!

Voted up and SHARED. Thanks for a great read.


rahul0324 profile image

rahul0324 4 years ago from Gurgaon, India

detailed research! The depth of the knowledge here is by far too deep to locate! The information is intriguing though! Suspicions about whether there is one prime LUCA or multiple LUCAS are not so easy to erase it seems!

A very interesting read! Up and awesome!


Millionaire Tips profile image

Millionaire Tips 4 years ago from USA

It is an interesting concept, trying to find our earliest ancestor. As I study genealogy, I try to find each ancestor one step at a time, and might get a DNA test to go back farther, but this is really getting much further back!


scottcgruber profile image

scottcgruber 4 years ago from USA Author

DzyMsLizzy: That's the point - LUCA had to be single celled. This is not the immediate ancestor of humans but the very distant ancestor of all life, including the amoeba.

As for seeding by an alien race, well, that would be panspermia - an interesting topic for another hub... :)


ronaknky profile image

ronaknky 4 years ago from Bengaluru

Seems like you have done a lot of hard work to publish this hub. The amount of research you have done is visible in the content of the article.


janiek13 profile image

janiek13 4 years ago from Florida's Space Coast

Good article, well written. It is an interesting theory.


phdast7 profile image

phdast7 4 years ago from Atlanta, Georgia

Interesting topic and well handled. Good Hub.


profile image

jenubouka 4 years ago

Great article, really did some digging. I have never heard of LUCA; however as the technology advances I am sure we will discover more truths or theories of ancient descendants. Awesome.


jblais1122@aol profile image

jblais1122@aol 4 years ago from Kansas City, Missouri, USA

Great job. Great subject. Thank you.


Healthy Pursuits profile image

Healthy Pursuits 4 years ago from Oregon

A very interesting read. Thank you! I love the whole idea of mitochondria being an independent life form that blended with the cell. As mitochondria are now a necessary part of us, does that mean we are really, to some extent, some kind of symbiotic life form?


scottcgruber profile image

scottcgruber 4 years ago from USA Author

I think one could argue that we are a symbiotic life form. Mitochondria are just one example, and a very ancient one at that. A more current example would be the bacteria, yeasts, and archaea that live in our guts and on our skin. There are an estimated 10^14 bacteria cells living in each human body, about ten times the number of human cells. We give them a nice place to live, they help us with digestion - that's definitely a symbiotic relationship.


Trish_M profile image

Trish_M 4 years ago from The English Midlands

Hi :)

Fascinating stuff!

Really good.

Luca ~ the controversial new Adam! :)


Julieonline profile image

Julieonline 4 years ago

Interesting reading, thank you for sharing this with all. Voted up.


Simone Smith profile image

Simone Smith 4 years ago from San Francisco

Wow! Your Hub has made me want to study this subject in greater depth. What a fun process of historical/biological mystery solving!

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