Evolution of the brain  The principles that govern the evolution of brain structure are not well understood. Brain to body size scales allometrically. Small bodied mammals have relatively large brains compared to their bodies whereas large mammals (such as whales) have smaller brain to body ratios. If brain weight is plotted against body weight for primates, the regression line of the sample points can indicate the brain power of a primate species. Lemurs for example fall below this line which means that for a primate of equivalent size, we would expect a larger brain size. Humans lie well above the line indicating that humans are more encephalized than lemurs. In fact, humans are more encephalized than all other primates. One approach to understanding overall brain evolution is to use a paleoarchaeological timeline to trace the necessity for ever increasing complexity in structures that allow for chemical and electrical signaling. Because brains and other soft tissues do not fossilize as readily as mineralized tissues, scientists often look to other structures as evidence in the fossil record to get an understanding of brain evolution. This, however, leads to a dilemma as the emergence of organisms with more complex nervous systems with protective bone or other protective tissues that can then readily fossilize occur in the fossil record before evidence for chemical and electrical signaling. Recent evidence has shown that the ability to transmit electrical and chemical signals existed even before more complex multicellular lifeforms. Fossilization of brain, or other soft tissue, is possible however, and scientists can infer that the first brain structure appeared at least 521 million years ago, with fossil brain tissue present in sites of exceptional preservation. Another approach to understanding brain evolution is to look at extant organisms that do not possess complex nervous systems, comparing anatomical features that allow for chemical or electrical messaging. For example, choanoflagellates are organisms that possess various membrane channels that are crucial to electrical signaling. The membrane channels of choanoflagellates’ are homologous to the ones found in animal cells, and this is supported by the evolutionary connection between early choanoflagellates and the ancestors of animals. Another example of extant organisms with the capacity to transmit electrical signals would be the glass sponge, a multicellular organism, which is capable of propagating electrical impulses without the presence of a nervous system. Before the evolutionary development of the brain, nerve nets, the simplest form of a nervous system developed. These nerve nets were a sort of precursor for the more evolutionarily advanced brains.