Dating has proven this genus lived from 40 to 45 million years ago in the middle Eocene. The genus Eosimias is unique because of the presence of primitive and derived traits. It provides new insight into the phylogenetic relationships between simians and prosimians (especially the phylogenetic position of the haplorrhine prosimian tarsiers). It can best be described as a likely tree dweller that relied on a steady diet of insects and nectar. It was probably a nocturnal, solitary creature.
Most eosimiid species are documented by unique or fragmentary specimens. This, as well as the strong belief that simians originated in Africa has made it difficult for many to accept the idea that Asia played a role in early primate evolution. Although some continue to challenge the anthropoid resemblances found in Eosiimidae, extensive anatomical evidence collected over the past decade substantiates its anthropoid status.
Eosimias sinensis (Chinese: 中华曙猿, "dawn monkey of China") was first discovered in China in 1994 by Christopher Beard. It was found in a mountain near Liyang City, Jiangsu province, China. It is the earliest catarrhine that has been discovered.
The species is believed to have lived 45 million years before present, in the Eocene epoch. E. sinensis was tiny, as small as the smallest monkey presently, the pygmy marmoset (Cebuella pygmaea) of South America, and could fit in the palm of a human's hand. Its teeth are considered more primitive than those of early higher primates known from Africa, including Algeripithecus. Due to its highly primitive nature, some paleontologists consider E. sinensis to be evidence that higher primates may have originated in Asia rather than Africa.
Christopher Beard was the lead member of the team that discovered Eosimias sinensis in 1994. Beard recovered a right mandible, cataloged as IVPP V10591, which preserved P4-M2 and roots or alveoli for C1, P2-3, and M3. Although it retains primitive characters such as a small body size (mean estimates range from 67–137 grams (2.4–4.8 oz)) and an unfused mandibular symphysis, it appears to be a primitive simian based on its dental characteristics, including a lower dental formula of 2-1-3-3.
Eosimias centennicus was found in 1995 while doing fieldwork in the Yuanqu Basin of the southern Shanxi Province in China. Among these recovered fossils is the first complete lower dentition of Eosimias, catalogued as IVPP V11000. All anatomical information yielded from these fossils confirms the anthropoid-like traits found in E. sinensis. Biostratigraphic evidence also suggests these fossils are younger than E. sinensis, which is consistent with the anatomy of eosiimids because the dentition of E. centennicus is slightly more derived than that of E. sinensis. This species was also found to be a very tiny primate, with mean estimates of body mass ranging from 91 to 179 grams (3.2 to 6.3 oz). E. sinesis was originally described on the basis of fragmentary fossils, but with the discovery of E. centennicus and a complete lower dentition, Eosimias can more definitively be described as an early anthropoid.