A study of coral suggests that ancient members of the animal kingdom slithered through the Precambrian mud with a hefty cache of genes in common with humans.

The coral Acropora millepora shares a surprisingly large number of genes with vertebrates. Credit: C. SÁNCHEZ

Surprisingly, many of these genes are not shared with creatures such as flies and worms, even though these animals evolved millions of years after coral. This calls into question some studies that use these model organisms to unravel the evolution of the human genome.

The investigation, published in this week's Current Biology (D. Kortschak et al. 13, 2190–2195; 2003), looked at some 1,300 gene sequences expressed in the coral Acropora millepora, and found that about 500 sequences had matches in gene databases. These sequences, called expressed sequence tags, represent either single genes, different pieces of the same gene, or expressed portions of DNA that do not contribute to a coding gene.

Of these, 90% were present in humans, and about 10% were found in humans but not in the fruitfly Drosophila melanogaster or the nematode worm Caenorhabditis elegans. This finding suggests that many genes thought to be vertebrate-specific may in fact have much older origins, and have been lost during the evolution of the fly and worm.

“The assumption was that coral would lack many of the genes found in higher animals,” says Robert Saint of the Australian National University in Canberra, one of the study's authors. Instead, they were surprised to find genes similar to those that contribute to the specialized tissues of vertebrate nervous systems, even though coral has only a simple nerve net.

“There are important basic scientific questions that we need to ask using the coral system that should tell us about the evolution of developmental mechanisms in the animal kingdom,” says David Miller, a molecular biologist at James Cook University in Townsville, Australia, and co-author of the study.

The idea that genes previously regarded as 'vertebrate innovations' may have evolved before vertebrates did isn't new. Alejandro Sánchez Alvarado, a developmental biologist at the University of Utah, previously found genes in the flatworm Schmidtea mediterranea that were thought to have evolved in vertebrates (A. S. Alvarado et al. Development 129, 5659–5665; 2002).

But the idea that some animals may discard genes as they become more sophisticated is still controversial. “We won't really know until we have more worm and insect genomes to compare,” says developmental biologist Eric Davidson of the California Institute of Technology in Pasadena.

The finding means that although fly and worm models are useful for studying gene function in development and cellular processes, they may be of limited value in studies of the evolution of human genes. “We need to look at many other animal genomes that haven't undergone the same degree of gene loss to understand the evolution and function of human genes, and how they generate complexity,” says Sánchez Alvarado.