Genes of the Hox cluster are restricted to the animal kingdom and play
a central role in axial patterning in divergent animal phyla(1). Desp
ite its evolutionary and developmental significance, the origin of the
Hox gene cluster is obscure. The consensus is that a primordial Hox c
luster arose by tandem gene duplication close to animal origins(2-5).
Several homeobox genes with high sequence identity to Hox genes are fo
und outside the Hox cluster and are known as 'dispersed' Hox-like gene
s; these genes may have been transposed away from an expanding cluster
s. Here we show that three of these dispersed homeobox genes form a no
vel gene cluster in the cephalochordate amphioxus, We argue that this
'ParaHox' gene cluster is an ancient paralogue (evolutionary sister) o
f the Hox gene cluster; the two gene clusters arose by duplication of
a ProtoHox gene cluster. Furthermore, we show that amphioxus ParaHox g
enes have co-linear developmental expression patterns in anterior, mid
dle and posterior tissues. We propose that the origin of distinct Hox
and ParaHox genes by gene-duster duplication facilitated an increase i
n body complexity during the Cambrian explosion.