How did single celled organisms evolve into multicellular animals?

Single celled organisms have less kinases and phosphatases than the metazoa (multicellular animals). It is reasonable to suppose that signaling information is limiting for organismal complexity and that it is necessary to obtain new signaling currency to enable more elaborate cellular and developmental events. Therefore, it is important to understand how and why new kinases and phosphatases have contributed to biological innovation. One class of kinases, the tyrosine kinases, has expanded greatly in the metazoa, with a striking jump at the onset of multicellularity. This sudden expansion is thought to have been driven by the acquisition of a highly effective, modular signaling system consisting of three components:

1. A writer: the tyrosine kinase
2. A reader: the SH2 domain that specifically binds to phosphotyrosine
3. An eraser: the tyrosine phosphatases

The tyrosine-kinase/SH2 module is a relatively recent innovation and its expansion coincides with the appearance of the metazoa. We hope to gain insight into the evolution of complexity by studying how cellular control systems have integrated this new signaling information. This project is a collaboration with Wendell Lim’s group at UCSF.