Our ancestor’s ‘leaky’ membrane answers big questions in biology

All life on Earth came from one common ancestor -- a single-celled organism -- but what it looked like, how it lived and how it evolved into today's modern cells is a four billion year old mystery being solved by researchers at UCL using mathematical modelling.

Findings published in PLOS Biology suggest for the first time that life's Last Universal Common Ancestor (LUCA) had a 'leaky' membrane, which helps scientists answer two of biology's biggest questions:

1. Why all cells use the same bizarre, complex mechanism to harvest energy

2. Why two types of single-celled organism that form the deepest branch on the tree of life -- bacteria and archaea -- have completely different cell membranes

The leakiness of the membrane allowed LUCA to be powered by energy in its surroundings, most likely vents deep on the ocean floor, whilst holding in all the other components necessary for life.

The team modelled how the membrane changed, enabling LUCA's descendants to move to new, more challenging environments and evolve into two distinct types of single-celled organism, bacteria and archaea, creating the deepest branch of the tree of life.

Bacteria and archaea share many common features such as genes, proteins and mechanisms of reading DNA, initially leading scientists to believe they were just different types of bacteria. Their classification changed in the 1970's after extreme differences were found in the way they replicate DNA and in the structure of their cell membrane. As they both stemmed from LUCA, scientists set out to find answers in the structure and function of LUCA's membrane.

Dr Nick Lane (UCL Biosciences) who led the study said, "I find this work just beautiful -- it constrains a sequence of steps going from the strange cell that seems to have been the ancestor of all life today, right through to the deep division between modern cells. From a single basic idea, the model can explain the fundamental differences between bacteria and archaea. Is it right? I'd like to think so, but more importantly, it makes some clear predictions that we plan to test in the future."

Data from the study strongly suggest that LUCA lived in the area where ancient seawater, dense with positively charged particles called protons, mixed with warm alkaline vent fluid, which contained few protons. The difference in the concentration of protons across these two environments enabled protons to flow into the cell, driving the production of a molecule called adenosine triphosphate (ATP) which powered the growth of cells, just as it does today.

owever, unlike modern cells the scientists believe this could only happen if the membrane was 'leaky', enabling protons to leave the cell spontaneously so more protons could enter to power growth.

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Our ancestor's 'leaky' membrane answers big questions in biology