Searching for LUCA, the first life-form from which all other life descended

Sub : Sci

Sec : Msc

Context:

In a new study, scientists have said the last universal common ancestor (LUCA) could have formed just 300 million years after the earth formed.

What is LUCA?

The last universal common ancestor (LUCA) is the hypothesized common ancestral cell from which the three domains of life, the Bacteria, the Archaea, and the Eukarya originated.
The concept of LUCA is based on the phylogenetic relationship among all the organisms.
LUCA is not the first living form on earth or the first living ancestors.
There is no fossil evidence to support the existence of LUCA, but the fact that modern genomes share so many features provides some insights.
The first universal common ancestor (FUCA) is a hypothetical non-cellular ancestor to LUCA and other now-extinct sister lineages.

Which is older: LUCA or fossils?

According to the journal Nature Ecology and Evolution, researchers at the University of Bristol and Exeter in the U.K. constructed a phylogenetic tree of 350 bacterial and 350 archaeal genomes and using a molecular clock, they estimated that LUCA could have originated around 4.2 billion years ago, just 300 million years after the earth itself formed.
The team also reported that LUCA may have had a small genome, of some 2.5 million bases and encoding around 2,600 proteins, which help it survive in a unique environmental niche.
The team also suggested the metabolites produced by LUCA — compounds produced as a result of its metabolism — could have created a ‘secondary’ ecosystem in which other microbes could have emerged.
Researchers have found fossil records of the earliest life-forms in the Pilbara Craton in western Australia, one of the few places on the planet where archaean rocks are exposed aboveground and accessible.
Studies of these fossils have suggested the life that lived on the rocks emerged around 3.4 billion years ago.
The current study has pushed this date back by almost a billion years, almost on the heels of the birth of our planet itself.
The researchers also found that LUCA may have had genes responsible for immunity, suggesting it had to fight off viruses.

What is a Molecular Clock?

The molecular clock is a figurative term for a technique that uses the mutation rate of biomolecules to deduce the time in prehistory when two or more life forms diverged.
The biomolecular data used for such calculations are usually nucleotide sequences for DNA, RNA, or amino acid sequences for proteins.
To calibrate the molecular clock to a particular rate of mutations, researchers establish links between a genome with known events, such as the ‘date’ on which the first mammal evolved or with the age of certain fossils. These links act like temporal benchmarks.

Origin of Earth

Earth formed around 4.54 billion years ago, approximately one-third the age of the universe.
There are a number of competing theories but all of them lack conclusive proof.
Oparin-Haldane hypothesis:
- In 1924 and 1929, Oparin and Haldane respectively suggested the first molecules making up the earliest life forms gradually self-organized from a “primordial soup” in a young earth’s tempestuous, prebiotic environment.
Miller-Urey experiment:
- Miller-Urey experiment in 1952, showed that in the right conditions, inorganic compounds could give rise to complex organic compounds.
- Miller and Urey mixed methane, ammonia, and water, and when they applied a strong electric current — like a lightning strike might have — the mixture contained amino acids, the building blocks of proteins.