Genes in us multicellular eukaryotes are characterized by a peculiar feature: the DNA sequence is interrupted by stretches called introns that are transcribed into mRNA, but then cut out so that their sequence is not represented in the final protein product. The gene is spliced together out of portions called exons, excluding the introns, a bit of post-transcriptional editing that permits splice variants to be made, and that can increase the diversity of gene products. It's still a very strange and inefficient way to go about making proteins, though, and one that isn't necessary—bacteria, for instance, get along just fine without this intron nonsense.

Now here's a paper on the comparative analysis of introns with a pair of surprises. Surprise #1: the introns in our genes are highly conserved, and about two thirds of the human introns examined were also present in our urbilaterian ancestors at the very same amino acid position and phase. What that means is that this peculiar disruption of our genes occurred in multicellular animals before the Cambrian, and we have preserved this quirk for half a billion years. While sequences have diverged, the way the genes are organized in blocks has been conserved.

Surprise #2 (although it shouldn't be): we vertebrates are primitive. Other clades have modified their gene structure over evolutionary history more than we have, and our genes are more similar to those of some obscure marine worms than they are to those of insects, for instance, and have changed less than those of some of our closer relatives.

The paper examined the organization of many genes common to all bilaterians, across the phylogenetic spectrum illustrated to the right: some insects, C elegans, humans, the pufferfish Fugu, the ascidian Ciona, and as a representative of the relatively less well studied Lophotrochozoa, the marine worm Platynereis (Platynereis is an interesting animal for other reasons, retaining a primitively diverse collection of eyes).

Using the metric of introns, the Ecdysozoa are a sophisticated bunch. On average in the sample used, their genes contain on average 2.5-5.4 introns each, which sounds like a lot until you compare it to humans: we have 8.4 introns per gene. You might wonder whether evolution has added more introns to the human lineage, or pared introns out of the Ecdysozoan lineage…and the answer can be found by looking at that marine worm group. Platynereis has 7.8 introns per gene, suggesting that we share the primitive condition and that the Ecdysozoa have undergone more extensive evolutionary modification of their gene structure. Similarly, Ciona has experienced a rapid burst of evolutionary change and in this one parameter is more different from us than we are from an annelid! (Before anyone freaks out and claims that this invalidates the cladogram above, keep in mind that this is looking at only a subset of the genes that are clearly orthologous between humans and Platynereis (30 genes) and is based on one feature, the location of introns.)

The data below show how Platynereis is more similar in this parameter to vertebrates than to other protostomes.

(A) Fraction of Platynereis introns present in other Bilateria. The scheme on the left indicates the phylogenetic position of Platynereis (solid circle) as well as the other species and the investigated internal nodes (gray arrows). Note that the value for Deuterostomia comprises all introns found in Ciona, Fugu, and humans and thus indicates the minimal fraction of Platynereis introns present in Urbilateria. (B) Fraction of human introns present in other Bilateria. The value for Protostomia includes all introns found in Ecdysozoa and Platynereis, thereby giving a minimal estimate of human introns present in Urbilateria.

Another way to look at it is to see the Ecdysozoa as shedding introns at a relatively rapid pace in their evolution. The diagram below illustrates this as the percentage of ancestral introns lost at each branch of the tree—these animals have eliminated 60-80% of their introns.

Most parsimonious scenario of intron losses in different branches of the Protostomia as inferred from the data set. Numbers designate the percent of ancestral protostome introns lost along the respective branches. All data have their basis in the evaluation of 30 randomly chosen Platynereis genes with orthologs in other species.

The bottom line is that we're, we're…well, we're just slow. We've been held back a few grades at the evolution school. We've been taking the short bus down the road of history.

We conclude that, at the intron and exon level, Platynereis and humans can be regarded as similarly slow-evolving representatives of protostomes and deuterostomes, respectively.

Not that there's anything wrong with that, of course.

We can be proud of our unique character, but one thing we aren't is the most highly refined species on the planet. That honor is reserved for the small, deft creatures most ruthlessly honed by luck and natural selection.

Raible F, Tessmar-Raible K, Osoegawa K, Wincker P, Jubin C, Balavoine G, Ferrier D, Benes V, de Jong P, Weissenbach J, Bork P, Arendt D (2005) Vertebrate-type intron-rich genes in the marine annelid Platynereis dumerilii. Science. 310(5752):1325-6.