Cited research: Genome Res. doi:10.1101/gr.100594.109 (2010)
The earliest stages of embryo development seem to be almost identical among mammals. However, Sheng Zhong at the University of Illinois at Urbana-Champaign and his team have found that 40.2% of the genes shared by humans, mice and cows are expressed differently at this point.
Their analysis of gene-expression patterns in embryos at various stages early in development showed that differences result from altered gene regulation. In some cases, mutations affected the binding of regulatory proteins. In others, transposons or 'jumping genes' had hopped in front of the genes, changing their regulation.
This variation among species suggests that multiple gene networks can guide embryo development, and could be harnessed to generate embryonic stem cells.