Motors, pumps, axles and gears made of molecules work in animal cells (including humans), and in bacteria and plants too. The molecules congeal thousands of atoms into machine-like parts. An oddity of evolution in one of these intricate machines is today’s post: it’s a random genetic accident 800 million years ago, with no beneficial effects, that became permanent.
The pump studied here flushes electrically charged atoms out of cells by a rapidly spinning rotor, like an electric motor’s. A ring made of proteins houses the rotor’s end, as in electric motors. In every living thing’s cells, two protein types (#1 and #2) make that ring, interlocked like jigsaw puzzle pieces. Pumps are pictured in different ways:
Photo credit (left): jpboseret.eu. (Right): email@example.com
There’s one exception. It’s a fungus which, 800 million years ago, started using three protein varieties – not two – to make its ring.
Why, the researchers asked, would this happen? 3-proteins would burden the organism: more ingredients, more complexity. But maybe overall it was an improvement? Maybe the 3-protein novel fungus survived better or reproduced faster than its 2-protein ancestor?
But no, it didn’t. It survived and reproduced identically.
How did the researchers look back 800 million years to figure this out? With great ingenuity. They manipulated genes of 3-protein ring cells so the altered cells constructed the 2-protein rotor ring. They reversed evolution’s clock, recreating the ancestors of that simpler era before 3-protein rings evolved. Then they checked how well those ancestors had functioned.
The weird surprise was that the custom-made ancestors worked equally well. They survived and reproduced identically to the natural 3-protein ring variety. The 3-protein complication brought no improvement.
The first question was: how did it happen? “Gene duplication” was the answer. Fungus cells proliferate by dividing. “Daughter” cells copy the genes of their “mother.” In the remote past, a daughter accidentally added an extra copy of a gene for one of the ring proteins. Say it was the gene for protein type #2.
Afterwards, for many generations, the twin genes produce identical #2 proteins, just more more of them. But over many generations, random mutations altered the once-twin genes. One began producing a new protein, type #3. Surprisingly, #3 still fit into the ring, jigsaw-puzzle style with protein types #1 and #2.
The second question was: why did this persist for 800 million years? The 3-protein complication was no better than the old 2-protein one. Why didn’t it just die out?
No one knows. Maybe it’s just a freak of evolution, a once-for-all-time exception that was passed down to offspring in trillions of generations without giving any benefit. This is surprising, and baffling, because evolution everywhere else seems ruthless in exterminating useless frills.
What phenomenon in nature?
Evolution produces complicated contraptions of molecular parts from simpler ancient ancestors.
What did this discovery show?
A mutation that conferred complication with no evolutionary advantage, but nevertheless was passed down to descendents for 800 million years.
What was known before?
What was believed before was that evolution kills off variations unless they confer a survival or reproductive advantage.
What remains unknown?
Are other useless ancestral changes perpetuated anyway? And what, if anything, does this reveal about how evolution actually works?
E. Finnigan et al., Evolution of increased complexity in a molecular machine, 481 Nature 360-364 (January 19, 2012); W.F. Doolittle, A ratchet for protein complexity, ibid, 270-271.