Strepsirrhini - Evolutionary History

Evolutionary History

Strepsirrhines include the extinct adapiforms and the lemuriform primates, which include lemurs and lorisoids (lorises, pottos, and galagos). The lemuriforms, and particularly the lemurs of Madagascar, are often portrayed inappropriately as "living fossils" or as examples of "primitive", "basal", or "inferior" primates. These views have historically hindered the understanding of mammalian evolution and the evolution of strepsirrhine traits, such as their reliance on smell (olfaction), characteristics of their skeletal anatomy, and their brain size, which is relatively small compared simian primates. In the case of lemurs, natural selection has driven this isolated population of primates to diversify significantly and fill a rich variety of ecological niches, despite their smaller and less complex brains compared to simians.

The origin of the earliest primates, from which both the strepsirrhines and haplorhines (simians and tarsiers) evolved, is a mystery. Both their place of origin and the group from which they evolved are uncertain. Although the fossil record demonstrating their initial radiation across the Northern Hemisphere is very detailed, the fossil record from the tropics—where primates most likely evolved—is very poor, particularly around the time that primates and other major clades (groups consisting of an ancestor and all its descendants) of eutherian mammals were first appearing. Consequently, geneticists and primatologists have used genetic analyses to determine the relatedness between primate lineages and the amount of time since they diverged. Using this molecular clock, divergence dates for the major primate lineages have suggested that primates evolved more than 80–90 mya, nearly 40 million years before the first primates appear in the fossil record.

The early primates include both nocturnal and diurnal small-bodied species, and all were arboreal, with hands and feet specially adapted for maneuvering on small branches. Plesiadapiforms from the early Paleocene are sometimes considered "archaic primates" because their teeth resembled those of early primates, and because they possessed arboreal adaptations, such as a divergent hallux (big toe). Although plesiadapiforms were closely related to primates, they may represent a paraphyletic group from which primates may or may not have directly evolved, and some genera may have been more closely related to colugos, also known as dermopterans or "flying lemurs", which are thought to be closely related to primates.

The first true primates (euprimates) do not appear in the fossil record until the early Eocene (~55 mya), at which point they radiated across the Northern Hemisphere during a brief period of rapid global warming known as the Paleocene–Eocene Thermal Maximum. These first primates included Cantius, Donrussellia, Altanius, and Teilhardina on the northern continents, as well as the more questionable (and fragmentary) fossil Altiatlasius from Paleocene Africa. These earliest fossil primates are often divided into two groups, adapiforms (sometimes called adapids, or adapoids) and omomyiforms (or omomyids—suspected relatives of tarsiers). Both appeared suddenly in the fossil record without transitional forms to indicate ancestry, and both groups were rich in diversity and were widespread throughout the Eocene. Although few fossils of extant primate groups—lemuriforms, tarsiers, or simians—are known from the Early to Middle Eocene, evidence from genetics and newer fossil finds suggest they may have been present during this early adaptive radiation. The divergence between strepsirrhines, simians, and tarsiers likely followed almost immediately after primates first evolved.