Fossil evidence is scarce for early stages of evolution in the ape family tree at the time before apes and the ancestors of humans diverged. A 13-million-year-old skull now offers insights into ape development at that time. See Article p.169
Perhaps because a rainforest habitat is a poor environment for fossilization, ape fossils are so rare that those of us who search for them are overjoyed when we find as much as an isolated tooth. Even the earliest known ape fossil, found in 25.2-million-year-old deposits in Tanzania, is only a single jawbone specimen1. Portions of fossilized ape skulls and limb bones are especially scarce, and have been discovered for only a handful of the dozens of ape species that lived in Africa during the Miocene epoch, between 23 million and 5 million years ago2. On page 169, Nengo et al.3 describe an extremely rare fossil find that I never thought would be made during my lifetime. This discovery will help to fill in missing information regarding the adaptations that influenced ape and human evolutionary histories, and shed light on long-standing mysteries about at least one enigmatic ape species.
Nengo and colleagues report their analysis of an ape skull (Fig. 1) that they found in deposits at Napudet in northern Kenya. Only one complete ape skull with an intact braincase (the region of the skull surrounding the brain) from the Miocene has previously been reported4, and that was a 7-million-year-old specimen from the genus Sahelanthropus, an ape that probably walked on two legs.
The Napudet fossil is convincingly dated as being 13 million years old. This was around the time when apes were beginning to expand their range into Eurasia5. Miocene fossil sites for this time are scarce in Africa, leaving us with little knowledge about ape evolution between 15 million years ago and the emergence of human ancestors that walked upright. In the context of these early fossils, human ancestors are defined as individuals on our branch of the evolutionary tree that had the capacity to walk upright, a grouping that could include direct human ancestors or human relatives.
The skull is that of an infant whose baby teeth had been knocked out, leaving behind only the baby molar roots. The authors used a technique called X-ray synchrotron microtomography to extract 3D images of developing adult teeth within the animal's jaw. These images are so detailed that the outlines and tracks of minute enamel secretions laid down at daily and 5-day intervals could be counted, enabling the authors to determine that the ape was 16 months old when it died. Shape analysis of the developing adult teeth reveals dental structures that are highly characteristic of apes belonging to a cluster of related species called the Nyanzapithecinae, a grouping that includes the earliest known fossil ape1. The authors classified their specimen as belonging to a new species that they named Nyanzapithecus alesi.
The infant's skull permits the first peek at a nyanzapithecine brain. Because the brains of modern apes develop much more rapidly than do those of humans, Nengo and colleagues interpret the infant's brain size as approaching that of an adult ape's. Size analysis of the skull and adult teeth indicate that if the infant had reached adulthood, it would have weighed about 11.3 kilograms at maturity. Relative to this estimated adult body weight, the brain cavity of N. alesi, 101 millilitres in volume, is substantially larger than the 35-ml brain-cavity volume of Old World monkeys from the same period6 (which form a branch on the evolutionary tree neighbouring the one containing humans and apes). These monkeys had brains closer in volume to those of modern lemurs of the same body mass. This scale of brain-size difference is greater than that existing today between Old World monkeys and apes.
The N. alesi skull has features similar to those of gibbons and great apes, laying to rest a previous suggestion that nyanzapithecines might be ape-like impostors that had branched off before the last common ancestor of the Old World monkeys and apes7. The N. alesi eardrum was positioned in a bony tube, an arrangement found in Old World monkeys and apes today. By contrast, in New World monkeys and primitive ape-like primates, including the extinct pliopithecids from the European Miocene, the eardrum is stretched across the surface of a circular ear opening.
Nyanzapithecus alesi probably looked remarkably similar to a baby gibbon, given its tiny mouth and nose relative to its head size, and the orientation of its cheek bones and eye sockets. Another similarity to gibbons is that its incisors developed faster than its molar teeth. However, Nengo et al. refrain from describing N. alesi as a direct gibbon ancestor. Gibbons have unusually long arms that help them to swing rapidly through the trees. Because the semicircular canals of a gibbon's inner ear are large relative to its body size, the animals perceive their environment as upright and stable, and can coordinate their body movements when their head position changes rapidly during locomotion8. By contrast, the semicircular canals of N. alesi are much smaller relative to its body size and are comparable to those of great apes, which move more slowly through trees than do gibbons.
In terms of its skull, N. alesi has several features in common with one of the strangest of all Miocene primates, the enigmatic and extinct Oreopithecus; samples of this animal were found in 8-million-year-old Italian deposits9, and its ancestral relationships and locomotor patterns remain controversial. The teeth of Oreopithecus are so unusual that it has been variously described as an ape, an Old World monkey, a primitive ancestor of Old World monkeys and apes, and a member of an extinct lineage unrelated to apes10. Although Oreopithecus is now considered a close relative of modern apes on the basis of its long arms, wide pelvis, short vertebral column and other features present in modern apes that swing through the trees11, it also has lower-limb traits found only in human ancestors that walked upright12,13,14. If its teeth were not so atypical for apes and human ancestors, suggestions that Oreopithecus was part of the human ancestral tree might have been embraced15. Instead, it is often considered to be an ape whose ability to walk upright evolved in parallel with that of the human ancestral lineage13,14.
It was initially suggested that Oreopithecus had an African origin, a proposal made on the basis of just a single tooth16 found in 15-million-year-old Kenyan deposits, and assigned to the nyanzapithecine species Mabokopithecus clarki. That tooth shares many of the same unusual and distinctive dental traits found in Oreopithecus. Nengo and colleagues' findings support that claim, interpreting the shape of the N. alesi skull and teeth as showing a close relationship to modern apes in general, and to Oreopithecus specifically.
However, the only limb bone known for Nyanzapithecus is a 15-million-year-old arm bone, and this lacked the adaptations that enable Oreopithecus and modern apes to swing through trees17. Consequently, arm adaptations for tree-hanging capacity might have evolved independently in the Oreopithecus lineage and in modern apes. Future discoveries of additional remains of N. alesi and other nyanzapithecines might reveal more about how this remarkable parallel evolution occurred. Further study of the N. alesi skull should provide additional insights about its relationships to other species.
Analysis of the skull markings should reveal how the N. alesi brain compares to that of other apes; it should also provide insight into the evolution of brain organization in primates. As an animal's brain grows, it pushes against the inside of the skull, leaving impressions on the bone of the curved, rope-like undulations known as gyri, and the deep fissures separating them, called sulci. Some of these patterns are characteristic of specific groups of related species. For example, Old World monkeys and great apes share an arrangement of gyri and sulci on the frontal lobe of the brain that is not found on the otherwise-complex frontal lobes of gibbons6. Such studies might untangle the complex relationship between nyanzapithecines, Oreopithecus and modern apes, as well as offer an opportunity to learn more about the adaptations that influenced the evolutionary histories of these fascinating animals.Footnote 1
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