…we’d know how DNA changes sculpted nature’s most complex invention — the human mind – in brains of hominid ancestors. Alas, 56,000 generations of human genomes – that’s more than a million years worth – are lost in the past. But 56,000 generations of another creature have had their genomes preserved. Gene by altered gene, they show evolution inventing a true novelty.
Why does this matter? Because it’s actual proof of what even Darwin admitted seemed almost miraculous, namely that tiny changes in (say) primitive, blind ancestors could produce descendants with complex eyes. Or could sculpt a human mind in ape-like hominids. “Show us proof,” say creationists. “Evolution is ‘only a theory’ till then.” The 56,000 generations are proof. This is how evolution happens.
The 56,000 generations have been captured since 1988. It’s a creature that (obviously) reproduces rapidly: an E. coli bacterium. More than six new generations burgeon every day from bacteria eye-droppered from yesterday’s test tube into a fresh supply of sugary liquid. For the last 25 years, a research team with Zen-like patience has seeded new generations every day, and retained frozen samples of the ancestors. And not just one lineage of ancestors and descendants. Twelve of them.
Evolution has acted, generation by generation, in the twelve lineages. Every change can be hunted down in the massive warehouse of stored test tubes.
The novelty that evolved was the ability to digest (“eat”) a chemical called citrate in the sugary solution even when oxygen is around. Ordinary E. coli can’t do that. Yes, as novelties go, this is less awesome than a human mind, but a novelty it truly is. Chemical tools that ordinary E. coli lack appeared in one of the twelve lineages. How, exactly?
These are some (and not even all) of the DNA changes found in test tubes of the mutant lineage. One affected a gene whose chemical product pulls citrate into the bacterium’s shell. That gene was copied twice, and both copies were joined in a tandem pair. Further, the tandem became controlled by a genetic on/off switch, one that is “on” when oxygen is present. The effect of all these DNA changes: even with oxygen present, citrate was consistently drawn into the shell of the mutants.
But not enough citrate for mutants to dominate. They were mere E. coli freaks, not a dominant new strain. Dominance required subsequent mutations, and these too came. Later generations copied the tandem duo, and copied it not just once, but up to nine times. (“Duplications” of genes can occur in cell division when the reproductive machinery splits apart the “mother” cell’s DNA helix and copies its genes for the “daughter” cell. Sometimes it makes extra copies of genes. That’s what happened here: “daughter” chromosomes contained up to nine copies of the tandem pair from their “mother” cells.)
What does this show? How random and chancy – and unlikely – is the evolution of novelties. They happen, but against long odds. Only one of the twelve lineages evolved the citrate-digesting power despite oxygen. The other eleven never did (or haven’t yet, anyway, in 56,000 generations).
At each step, chance could have aborted the novelty. Gene duplication on chromosomes is a random event. 30,000 generations went by without the tandem pair arising in the mutant lineage. But then, it did.
Not only did the tandem copying happen, but it also happened to put the tandem citrate-transporting gene under the control of a genetic switch that remained “on” with oxygen present. This too: random…chancy.
And yet even that was not enough for the novelty to dominate in populations of E. coli. Dominance came in later generations with the nine-fold duplications of the genetic tandem, a spate that finally drew in enough citrate for the descendants to out-compete “normal” E. coli strains with oxygen present.
The genetic pedigree of the new species is exactly mapped. Evolution was captured in action.
What phenomenon in nature?
Evolution: ancestors produce descendants with novel qualities that ancestors lacked.
What was known before?
That it happens, but few examples that map every step of the evolution.
What did this discovery show?
Every change in the genomes from ancestors that lacked the novelty to descendants that possessed it. And how low are the probabilities of novelties arising.
What remains unknown?
Many things. For instance, other examples of evolution with such impressively concrete evidence. Especially gratifying would be whatever genomic analysis can deduce about evolution of animals more charismatic and complicated — dolphins, peacocks, humans — than microscopic E. coli.
In this video, the lead author describes the work. http://www.youtube.com/watch?
Z. Blount et al., Genomic analysis of a key innovation in an experimental Escherichia coli population, 489 Nature 513-518, 27 September 2012; and H. Hendrickson and P. Rainey, How the unicorn got its horn, 489 Nature 504-505 27 September 2012.