Posts Tagged ‘Averroes’
photo: Minden/plainpicture (source)
Many biologists have long believed that, before the point 2.9 billion years ago that the three domains of life emerged, there was no speciation– genetic material of all sorts was freely exchanged in every direction. In this “pre-Darwinian” period, which lasted hundreds of millions of years, there was no “evolution”; rather, cells struggling to survive on their own exchanged useful parts with each other without competition.
Now, as a function of the effort to identify the Last Universal Common Ancestor (LUCA)– the organism from which all life on earth must be descended– scientists have begun to suspect that all of those cells trading parts were part of a single entity: an enormous mega-organism that filled the planet’s oceans before splitting into three and giving birth to the ancestors of all living things on Earth today.
It was around 2.9 billion years ago that LUCA split into the three domains of life: the single-celled bacteria and archaea, and the more complex eukaryotes that gave rise to animals and plants (see timeline). It’s hard to know what happened before the split. Hardly any fossil evidence remains from this time, and any genes that date that far back are likely to have mutated beyond recognition.
That isn’t an insuperable obstacle to painting LUCA’s portrait, says Gustavo Caetano-Anollés of the University of Illinois at Urbana-Champaign. While the sequence of genes changes quickly, the three-dimensional structure of the proteins they code for is more resistant to the test of time. So if all organisms today make a protein with the same overall structure, he says, it’s a good bet that the structure was present in LUCA. He calls such structures living fossils, and points out that since the function of a protein is highly dependent on its structure, they could tell us what LUCA could do…
LUCA had a rich metabolism that used different food sources, and it had internal organelles. So far, so familiar. But its genetics are a different story altogether. For starters, LUCA may not have used DNA. Poole has studied the history of enzymes called ribonucleotide reductases, which create the building blocks of DNA, and found no evidence that LUCA had them (BMC Evolutionary Biology, DOI: 10.1186/1471-2148-10-383). Instead, it may have used RNA: many biologists think RNA came first because it can store information and control chemical reactions.
The crucial point is that LUCA was a “progenote“, with poor control over the proteins that it made, says Massimo Di Giulio of the Institute of Genetics and Biophysics in Naples, Italy. Progenotes can make proteins using genes as a template, but the process is so error-prone that the proteins can be quite unlike what the gene specified. Both Di Giulio and Caetano-Anollés have found evidence that systems that make protein synthesis accurate appear long after LUCA. “LUCA was a clumsy guy trying to solve the complexities of living on primitive Earth,” says Caetano-Anollés…
Only when some of the cells evolved ways of producing everything they needed could the mega-organism have broken apart. We don’t know why this happened, but it appears to have coincided with the appearance of oxygen in the atmosphere, around 2.9 billion years ago. Regardless of the cause, life on Earth was never the same again.
As we rethink the roots of our family trees, we might spare a thought for the Spanish-Arab philosopher, physician, and astronomer known in the West as Averroes; he died on this date in 1198. The most famous of medieval Muslim philosophers, he was an authority on Aristotle, whose thought he defended against the charge that it was an affront to Islam. His Kulliyat fi ab tb (Generalities on Medicine) attempted to codify logically all existing medical knowledge– from organ anatomy and hygiene to the prevention, diagnosis, and treatment of diseases– and spread widely via translations. In astronomy, he argued for strictly concentric orbital organization, believing that the motion of the planets had to be around a physical center (the Earth)– thus rejecting Ptolemy’s system of epicycles.
Two out of three ain’t bad.