Many people think of evolution purely as a march towards progress and complexity, but evolution is an engine with several gears. For example, a population can refrain from evolving, remaining practically unchanged for millions of years. Or it can become less complex, cutting away cumbersome genes and structures. Publishing in the journal Science, a group of international researchers have discovered that when it comes to gene structure, humans have barely evolved at all, as our genes have retained the high intron number of one of our ancient ancestors. Surprisingly, some of this complexity has been lost in simpler organisms with faster reproduction. As the press release says, this "overturns a commonly-held view of the nature of genes in the first animals."

Animal genes contain pieces of DNA called introns, which are excised before the protein is produced. This might sound cumbersome, but introns add flexibility, allowing the cell to stitch different pieces together, letting it produce several different proteins from the same gene. Humans have a lot of introns in their genes, whereas fruit flies and nematodes have fewer, and it has been assumed that the simple genes reflect a more primitive condition. But the new study changes that.

The common ancestor of fruit flies, nematodes, and humans is called Urbilateria (see the figure on the right for a family tree). Urbilateria is no longer around for us to study, so the researchers looked at an organism that had retained many of its features: The annelid Platynereis dumerilii. Annelids are an ancient group, and Platyneriis lives in an environment similar to that which Urbilateria is thought to have occupied.

From the genes shared by both fruit flies, nematodes, annelids, and humans, the researchers picked 30 genes at random, and analyzed their intron content. It was discovered that humans shared at least two thirds of their introns with Urbilateria, whereas nematodes and fruit flies had lost 77% and 84% of the urbilaterian introns, respectively. As the authors conclude:

Our analyses consistently support the notion that Urbilateria possessed genes that, in both structure and sequence, were more similar to today's human or Platynereis genes than to those of dipterans, nematodes, or ascidians, where these initially complex genes have been secondarily simplified.

Just this year, two other articles have shown reduction to have played a surprisingly important role in evolution. Of course, all of this concerns multicellular animals, and as I've explained before, we really don't have much of a handle on single-cellular organisms. But I'll make a bold prediction: Once we start sequencing the oldest branches of our family tree, we will see that we have inherited more of our complexity from the microcopes than previously thought.

References

Raible F., et al., "Vertebrate-Type Intron-Rich Genes in the Marine Annelid Platynereis dumerilii", Science 310(5752):1325 - 1326 (2005)

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