Last update: August 15, 2025

The Origin of Life: From the First Organisms to Current Biological Diversity

Phylogenetic tree of life showing LUCA and the three domains of life

LUCA: The Last Universal Common Ancestor

The last universal common ancestor, or LUCA, represents the hypothetical organism from which all current living beings on Earth descend. Research suggests that LUCA lived about 3.5 to 3.8 billion years ago, shortly after the formation of the oceans. Comparative genomic studies have identified about 355 genes that were probably present in LUCA.

Table of Diversification into Three Domains

From LUCA, life diversified into three major domains: Bacteria, Archaea, and Eukarya. Molecular analyses show that Archaea and Eukarya share a more recent common ancestor with each other than with Bacteria.

Table of Compared Characteristics of the Three Domains of Life
Characteristics	Bacteria	Archaea	Eukarya
Cell type Defines the basic structure of the cell	Prokaryote	Prokaryote	Eukaryote
Nucleus The nucleus contains DNA in eukaryotes	❌ No	❌ No	✅ Yes
Membrane-bound organelles Ex. mitochondria, endoplasmic reticulum	❌ No	❌ No	✅ Yes
Cell membrane Type of lipids that make up the membrane	Ester lipids Less stable, typical of bacteria	Ether lipids Very stable, adapted to extreme environments	Ester lipids Like bacteria, but with cholesterol
Cell wall Rigid structure surrounding the cell	Peptidoglycan Polymer specific to bacteria	Pseudo-peptidoglycan Similar structure but chemically different	Cellulose, chitin, or absent Varies by type of eukaryote
Ribosomes Sites of protein synthesis	70S Small, typical of prokaryotes	70S Similar to bacteria but with subtle differences	80S Larger, specific to eukaryotes
Intronic genes Presence of introns in genes	❌ Rare Compact genome	✅ Sometimes Similarities with eukaryotes	✅ Frequent Introns present in most genes
Genetic organization Form of DNA in the cell	Circular DNA	Circular DNA	Linear DNA
Reproduction Mode of cell multiplication	Asexual By binary fission	Asexual Often by fission	Sexual or asexual Meiosis, mitosis, or simple division
Habitats Natural environment	Everywhere Soil, water, living organisms	Extreme environments Hot springs, saline, acidic	Varied environments Terrestrial, aquatic, aerial
Examples Representative organisms	Escherichia coli, Streptococcus	Halobacterium, Methanopyrus	Humans, plants, fungi

The Complex Evolution of Eukaryotes

The appearance of eukaryotic cells marks one of the major turning points in the history of life. Unlike prokaryotic cells (bacteria and archaea), eukaryotes have a nucleus, specialized organelles (such as mitochondria), and a sophisticated cellular organization. This complexity allowed the emergence of plants, animals, fungi, and all multicellular life.

Origins and Ancestors

Eukaryotes probably resulted from an endosymbiosis between an archaeon and a bacterium, giving rise to mitochondria. This symbiosis enabled the development of more complex and larger cells.

LECA (Last Eukaryotic Common Ancestor): This last ancestor already had mitochondria, a nucleus, and a cytoskeleton.
Asgard Archaea: Recently discovered, they are considered the closest relatives of eukaryotes. They possess genes similar to those of modern eukaryotes.
Endosymbiosis: Theory that an ancestral cell absorbed a bacterium, which then became a mitochondrion. This process was crucial for the evolution of eukaryotes.

The Molecular Clock and Dating

Definition and Origin of the Concept

The molecular clock is a hypothesis that genetic mutations accumulate at a relatively constant rate in the genome over time. This concept, proposed in the 1960s by Zuckerkandl and Pauling, allows dating the divergence between species by comparing their DNA or protein sequences.

N.B.: The more different the genetic sequences of two species are, the older their common ancestor, and vice versa.

Operating Principle

Constant mutation: Changes in biological macromolecules (DNA, proteins) occur at an average frequency.
Sequence comparison: The number of differences between two species is measured.
Paleontological calibration: These differences are associated with known dates thanks to fossils.

Applications

Dating the divergence between species: For example, estimating when humans and chimpanzees diverged.
Reconstruction of the tree of life: Helps to better understand evolutionary relationships.
Complement to fossils: Useful when the fossil record is incomplete.

Limitations and Precautions

The mutation rate is not always constant; it can vary depending on species, genes, or environments.
Calibration depends on the quality of available fossil data.
Molecular clocks are more reliable over long periods than over recent times.

Source: Nature Microbiology - The nature of the last universal common ancestor and Science - Asgard archaea illuminate the origin of eukaryotic cellular complexity.