Aside from the brain, the liver is the most complex organ in the body. This irreducibly complex structure has over 500 critical functions in humans. A vertebrate cannot survive without a liver. Invertebrates do not have a liver or anything close, and an enormous unbridgeable gap exists between invertebrates and vertebrates in this area. As many of its functions are required for life, structures and cells exist in invertebrates that perform some of liver functions. Furthermore, the liver in all mammalian species is close to identical, and no evidence exists of any evolution of the liver. The progenitor to mammal liver was an organ close to identical to the liver in modern humans. Likewise, some mammals have a gallbladder connected associated with the liver, while others lack this support system, clearly showing that the first gallbladder was a fully functional organ which included all of its support systems.