LUCA: The Great-Grandparent of Life
Life on earth is typified by bewildering complexity and diversity. In Nature, uniqueness seems to be a commodity in oversupply. The sombre subject of taxonomy is burdened with enough ambiguity and supposition to be considered an art. At first glance, there seems to be little to suggest that such a gamut of creatures originated from one life form.
However, religious edicts and philanthropic sermons preaching the unity of life may not be far from the truth. Life is one, in the sense that all life forms alive today are descended from the same organism, dubbed LUCA, the Last Universal Common Ancestor.
Although a bacterial cell and a human cell may not appear to share much in common, there are implicit similarities. The universality of DNA as the genetic code, and the omniscience of the genetic dictionary for proteins imply a common source. For example, the genetic code UUU specifies the amino acid Phenylalanine in all living creatures, from bacteria to fish.
LUCA precedes the split between archaea, prokaryotes and eukaryotes. Its structure is shrouded in mystery, and is made even more elusive due to the complications of lateral gene transfer. Lateral gene transfer makes it difficult for researchers to determine which genes in the genomes of all living things date back to LUCA.
LUCA Had a Complex Cell Structure
Many scientists believe that LUCA was little more than a rudimentary assemblage of molecular parts, a chemical soup out of which evolution gradually constructed more complex cells. Some question whether it was even a cell.
However, according to a study published in the journal Biology Direct, LUCA may have been more complex than was previously thought, with a sophisticated structure recognizable as a cell.
The study identifies a formerly overlooked storage site of polyphosphates, a type of energy currency in cells, as an organelle in microbial cells.
Organelles are similar to organs in the body, and were thought to be absent in bacteria and their distantly related microbial cousins, the archaea.
The new evidence indicates that this storage site actually represents the first universal organelle, which is common to all the three domains of life.
LUCA: A Planetary Super-Organism?
Other research suggests that LUCA may have been a planetary mega-organism.
Instead of competing for resources and developing into separate life forms, cells exchanged genetic material freely for hundreds of millions of years. This allowed species to obtain the tools to survive without competition.
“It was more important to keep the living system in place than to compete with other systems,” says Caetano-Anollés of the University of Illinois at Urbana-Champaign. He asserts that the free exchange and lack of competition implied that this living primordial ocean essentially functioned as a single mega-organism.
If this is true, evolution operated in a very different way during the reign of LUCA.
Useful parts, or helpful proteins, could spread from one cell and become globally widespread. Harmful or less efficient proteins were discarded and flensed by natural selection.
The mega-organism could have broken apart only when some of the cells evolved ways of producing everything that they needed. It is not known why this split happened, but it appears to have coincided with the appearance of oxygen in the atmosphere, about 2.9 million years ago. This kick-started evolution as we know it, paving the way for the dawn of the myriad species alive today.
Lateral Gene Transfer or Horizontal Gene Transfer- It refers to the transfer of genes in a manner other than traditional reproduction. It differs from Vertical Gene Transfer, which is the transmission of genetic information from the parent to the offspring.
Evolution- The process of change in a species or population, over many generations, is evolution. It is governed by the principles of natural selection and the ‘survival of the fittest’.