Previous genome comparisons have suggested that one important trend in vertebrate evolution has been a sharp rise in intron abundance. By using genomic data and expressed sequence tags from the marine annelid Platynereis dumerilii , we provide direct evidence that about two-thirds of human introns predate the bilaterian radiation but were lost from insect and nematode genomes to a large extent. A comparison of coding exon sequences confirms the ancestral nature of Platynereis and human genes. Thus, the urbilaterian ancestor had complex, intron-rich genes that have been retained in Platynereis and human.

The marine ecosystem is governed by a multitude of environmental cycles, all of which are linked to the periodical recurrence of the sun or the moon. In accordance with these cycles, marine species exhibit a variety of biological rhythms, ranging from circadian and circatidal rhythms to circalunar and seasonal rhythms. However, our current molecular understanding of biological rhythms and clocks is largely restricted to solar‐controlled circadian and seasonal rhythms in land model species. Here, we discuss the first molecular data emerging for circalunar and circatidal rhythms and present selected species suitable for further molecular analyses. We argue that a re‐focus on marine species will be crucial to understand the principles, interactions and evolution of rhythms that govern a broad range of eukaryotes, including ourselves.