A study published in Proceedings of the National Academy of Sciences claims to have solved this scientific riddle by analysing the genomics of primitive living fishes such as sharks and lampreys and their spineless relatives, such as the sea squirts.

Vertebrates - animals such as humans that possess a backbone - are the most anatomically and genetically complex of all organisms, but explaining how they achieved this complexity has vexed scientists since the conception of evolutionary theory.

Alysha Heimberg of Dartmouth College and her colleagues showed that microRNAs, a class of tiny molecules only recently discovered residing within what has usually been considered ‘junk DNA’, are hugely diverse in even the most lowly of vertebrates, but relatively few are found in the genomes of our invertebrate relatives.

She explained: “There was an explosive increase in the number of new microRNAs added to the genome of vertebrates and this is unparalleled in evolutionary history.”

Co-author, Dr Philip Donoghue of Bristol University’s Department of Earth Sciences continued: “Most of these new genes are required for the growth of organs that are unique to vertebrates, such as the liver, pancreas and brain. Therefore, the origin of vertebrates and the origin of these genes is no coincidence.”

Dr Kevin Peterson of Dartmouth College said: “This study not only points the way to understanding the evolutionary origin of our own lineage, but it also helps us to understand how our own genome was assembled in deep time.”

This work was funded by the National Science Foundation, and the National Endowment for Science, Technology and the Arts.

Paper: MicroRNAs and the advent of vertebrate morphological complexity by Alysha M. Heimberg, Lorenzo F. Sempere, Vanessa N. Moy, Philip C. J. Donoghue and Kevin J. Peterson will be published online on February 11-15 in PNAS.