The last universal common ancestor, rather than a primitive blob of chemicals, likely was more complex, even having so-called organelles or miniature organs. CREDIT: David Huntley | Shutterstock View full size image

The mysterious common ancestor of all life on Earth may have been more complex than before thought — a sophisticated organism with an intricate structure, scientists now suggest.

The last universal common ancestor, or LUCA, is what researchers call the forerunner of all living things. Much about LUCA remains enigmatic — many think it was little more than a primitive assemblage of molecular parts, a chemical soup from which evolution gradually built more complex forms. Some even debate whether it was even a cell.

Now, after years of research into a once-neglected feature of microbes, scientists suggest the last universal common ancestor was indeed complex, and recognizable as a cell.

Miniature organs

The researchers focused on a region of cells loaded with high concentrations of polyphosphates, molecules such as ATP used to transfer energy around the cell in chemical form. This storage site for polyphosphates may represent the first known universal organelle — compartments within cells that essentially act as miniature organs — the investigators suggest. Other kinds of organelles include the chloroplast, which gives plants the ability to use sunlight as energy, and the mitochondrion, which allows life to use oxygen for respiration.

Scientists had thought organelles were absent from bacteria and their distantly related microbial cousins, the archaea. Now these findings suggest this polyphosphate storage organelle is present in all three domains of life — bacteria, archaea and the eukaryotes, which include animals, plants and fungi.

“It was a dogma of microbiology that organelles weren’t present in bacteria,” said researcher Manfredo Seufferheld, a stress physiologist and cell biologist at the University of Illinois at Urbana-Champaign. Still, earlier research of his and his colleagues’ showed that the polyphosphate storage structure in at least two bacterial species was physically, chemically and functionally the same as an organelle called an acidocalcisome found in many single-celled eukaryotes.

To look for this storage unit, in their latest research the team analyzed the evolutionary history an enzyme known as a vacuolar proton pyrophosphatase (V-H+PPase), which is common in the acidocalcisomes of eukaryotic and bacterial cells. The results showed archaea also have the enzyme and a structure with the same physical and chemical properties as an acidocalcisome.

“This organelle appears to be universal,” Seufferheld told LiveScience. “This suggests the last universal common ancestors had a lot more cellular structure than others had thought.”

Describing a common ancestor

By comparing the sequences of the genes for this enzyme from hundreds of organisms representing the three domains of life, the researchers constructed a family tree showing how different versions of the enzyme in different species were related. The more similar sequences were, the more closely they were related, and the less similar they were, the more distantly they were related.

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