What is LUCA? – The Hindu

LUCA’s identity

The last universal common ancestor, known by its shortened form as LUCA, has been a topic of research and debate for a long time. It is a commonly known fact that the cell is the basic unit of all life forms. Every living creature in this world, from the smallest ant to the largest whale shark, is made up of cells. Well, the LUCA has been hypothesised to be the one common ancestor for all these organisms! It is defined as a single-celled organism from which all three domains of life – bacteria, archaea, eukaryotes – have emerged. If one could witness the endless family branches of all the living beings in the world, they would see LUCA at the very root of what is known as the ‘Tree of Life’. The Tree of Life is a conceptual model in biology that is used as a tool for studying and researching evolution and exploring the relationship and connection between abundant, endangered, and extinct organisms. It was first described in a popular passage in Charles Darwin’s On the Origin of Species, which also spoke of the theory of one organism being the sole ancestor of all creatures. 

“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.”

Genes and LTG

Studies have shown that the LUCA existed about 4 billion years ago. In old studies done on this organism, researchers mainly looked for genes that were shared commonly amongst all genomes (the complete set of an organism’s DNA). The speculation was that if one gene could be found as a common trait in all forms of life, then it came from LUCA. However, the phenomenon known as LTG (latent gene transfer) seemed to bring this theory to a standstill, since LTG was seen to be the cause for the presence of over 11,000 common genes, meaning that just because a gene was common, it didn’t necessarily mean that it came from LUCA. LTG is the transfer and movement of genetic material between organisms through processes like conjugation and transduction. However, knowing this, evolutionary biologist William Martin and his team began a search for particularly ancient genes that held extremely long lineages, but also held evidence that they weren’t shared or transferred by LTG. At the end of the research and analysis conducted, Martin and his team confirmed that 355 of the 11,000 genes belonged to LUCA and could provide an insight into how the organism lived. Evidence has been uncovered that the single-celled organism dwelled deep underground in an environment rich in iron and sulpher. One prime example would be deep-sea hydrothermal vents.

Appearance and theories

Since there are no fossils of LUCA, scientists have not been able to provide a proper visual of the single-celled organism. However, through the study of genes, scientists have been able to make educated speculations, theories, and statements on what composed its body. Based on studies conducted, it is said to have a lipid bilayer,which is a continuous barrier around the cell that protects the ions and proteins; ribosomes, which are molecular machines found in all cells, and a ring-shaped coil of DNA that floated freely within it. 

Bacteria
| Photo Credit:
AP

Bacteria: Bacteria are free-living prokaryotic microorganisms that can be found everywhere on Earth, from soil and water to geothermal hot springs and radioactive waste. A bacterium lacks a membrane-bound nucleus and plays an important role in the recycling of nutrients. A soil-dwelling bacterium called geosin is the cause of the petrichor or the earthy smell that arises after rain.

Archaea
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WIKIMEDIA COMMONS

Archaea: Archaea are a group of prokaryotic microorganisms that can thrive in the most extreme of environments, for which they were called extremophiles. Similar to bacteria, they also lack a membrane-bound nucleus. They are abundantly found in the ocean’s plankton.

Eukaryotes
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FREEPIK

Eukaryotes: Eukaryotes are organisms whose cells hold a membrane-bound nucleus, making them different from the former two. Eukaryotes can be both multicellular and unicellular. It comprises animals, plants, and fungi, and differentiates from bacteria and archaea with its size and internal structure.

GLOSSARY

Conjugation: A type of horizontal gene transfer where bacteria transfer genetic material to one another through physical contact. 

Transduction: The process by which DNA is transferred from one bacterium to another through a virus.