Why do scientists feel hox genes are evidence that all organisms ultimately share a common ancestor.

Hox gene are basically the controlling gene which controls the timing and growth of embryonic development with presence in every animal kingdom species.

Explanation:

The presence of Hox gene in every animal kingdom species is self explanatory to have a common link between them all, even including the humans.

The role of Hox gene is a controlling gene. It controls the time of embryonic development that is, when to start dividing and which way of division should carry on for how long. This let the deciding factor for the growth of the embryo, and comes the variation, and hence, evolutionary pattern can be sketch out. This is why the scientists feel its evidentiary value in all organisms sharing common ancestor.