The image below shows HOX genes (homeotic genes) and how they regulate the body structure of both a fly and a human.
All eukaryotes evolved from a common ancestor with these genes and, while they have not remained identical and have evolved over time, their essential functions have remained relevant thus they have been retained.
You can learn more about homeotic genes in general here and you can read about homeotic genes and body structure here.
"Hox genes (also known as homeotic genes) are a group of related genes that control the body plan of an embryo along head-tail axis.
After the embryonic segments have formed, the Hox proteins determine the type of segment structures (e.g. legs, antennae, and wings in fruit flies or the different types of vertebrae in humans) that will form on a given segment. They appear to be controlling genes.
Deﬁnitive proof for a genetic basis of some homeotic changes was obtained by isolating homeotic mutants.
The ﬁrst homeotic mutant was found by Calvin Bridges in Thomas Hunt Morgan's laboratory in 1915.
This mutant shows a partial duplication of the thorax and was therefore named Bithorax (bx). It transforms the third thoracic segment (T3) toward the second (T2)."
This organism is still being bred to day and can be ordered by catalog.