Last Universal Common Ancestor – LUCA

Theories of origin of life on Earth

Many theories have been propounded by scientists on the origin of earth. However, there is no consensus on the exact mechanism of origin of life on earth.

• Oparin-Haldane Hypothesis: Alexander Oparin and J. B. S. Haldane independently proposed that the first molecules making up the earliest life form gradually self-organised from a ‘primordial soup’ in a young earth’s difficult, prebiotic environment. 
• Miller-Urey experiment: Stanley Miller and Harold Urey from the University of Chicago showed that in right conditions, inorganic compounds could give rise to complex organic compounds. They mixed methane, ammonia and water and then applied a strong electric current – like a lightning strike. The result of this experiment was that the mixture contained amino acids, which are building blocks of proteins. 
• Meteoritic origin: Some scientists believe that meteorites from space could have brought buildings blocks of life, which was sustained by Earth. In 2019, French and Italian scientists reported discovering extra-terrestrial organic material 3.3 billion years old. Japan’s Hayabusa 2 mission to asteroid Ryugu indicated the presence of more than 20 amino acids there.

About Last Universal Common Ancestor (LUCA)

• There has also been a lot of curiosity among the researchers on the evolution of life forms and the rich diversity that they have resulted into. 
• Scientists believe that all three branches of life – bacteria, archaea and eukarya – originated from a single cell, called the last universal common ancestor (LUCA).
• There is no fossil evidence to support the existence of LUCA.

Evidence in support of LUCA:

• Modern genomes share many features.
• Theory of Molecular Clock: 
  • Scientists reconstruct the ‘tree of life’ using the theory of molecular clock. According to this theory, the rate at which mutations are added or removed from a population’s genome is proportional to the rate of acquiring new mutations ie genetic mutations in the genetic material occur at a relatively constant rate over time. The rate of mutations also varies between species. 
  • Using this, researchers have developed a way to estimate how much time could have passed between two evolutionary events. Links between a genome and known events like the date of origin of first mammals is used to calibrate the molecular clock.
  • Researchers from university of Bristol and Exeter in UK have constructed a phylogenetic tree of 350 bacterial and 350 archaeal genomes. Using a molecular clock, the team estimated LUCA could have been originated around 4.2 billion years ago, just 300 million years after the birth of earth.
  • These are about one billion years older than earliest fossil evidence found on earth at Pilbara Craton in Western Australia. 
  • According to researchers, LUCA had a small genome with only 2.5 million bases which encoded around 2,600 proteins, just enough to help it survive in a unique environmental niche. 
  • 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. 

Earliest fossil evidence

• Researchers have found fossil records of the earliest life-forms in the Pilbara Craton in western Australia, which is one of the few places on the planet where archaean rocks are exposed on ground.
• These fossils are organisms which lived around 3.4 billion years ago.
• The origin of LUCA is around a billion years ago from this earliest fossil evidence.

Significance of the study:

• Contributes significantly to our understanding of origin and evolution of life on earth.
• Enables us to look for similar forms of life across the universe.
• Provides fillip to our abilities to engineer synthetic organisms of various industrial, chemical and biological processes on Earth and create or moderate ecosystems on other planets.