One of the hallmark characters of animals is the presence of a specific cluster of genes that are responsible for staking out the spatial domains of the body plan along the longitudinal axis. These are the Hox genes; they are recognizable by virtue of the presence of a 60 amino acid long DNA binding region called the homeodomain, by similarities in sequence, by their role as regulatory genes expressed early in development, by the restriction of their expression to bands of tissue, by their clustering in the genome to a single location, and by the remarkable collinearity of their organization on the chromosome to their pattern of expression: the order of the gene’s position in the cluster is related to their region of expression along the length of the animal. That order has been retained in most animals (there are interesting exceptions), and has been conserved for about a billion years.
Think about that. While gene sequences have steadily changed, while chromosomes have been fractured and fused repeatedly, while differences accumulated to create forms as different as people and fruit flies and squid and sea urchins, while continents have ping-ponged about the globe and meteors have smashed into the earth and glaciers have advanced and retreated, these properties of this set of genes have remained constant. They are fundamental and crucial to basic elements of our body plan, so basic that we take them completely for granted. They determine that we can have different regions of our bodies with different organs and organization. Where did they come from and what forces constrain them to maintain their specific organization on the chromosome? Are there other genes that are comparably central to our organization?