Chimpanzees and Humans Share a Common Ancestor that Lived in the Late Miocene

Monday, May 24, 2021

To understand the origins of our lineage it is necessary to reconstruct the morphology, behavior and environment of the last common ancestor between modern humans and other apes. However, there is no scientific consensus on the phylogenetic positions of the diverse and widely distributed Miocene apes.

These fossil apes are often at the center of the debate, as some scientists dismiss their importance in the origins of the human lineage (the “hominins”) and others confer a star role in evolution.

According to a study published in the journal Science, these prehistoric remains can inform us about essential aspects of evolution, including the nature of our last common ancestor: a species of ape unlike any currently alive.

The researchers reviewed the main theories about the origin of the human lineage in the Miocene, as well as the evolutionary role of the apes of that period, since the publication of “The origin of the man” 150 years agoDarwin, 1871). The work includes discoveries in the fields of comparative anatomy, paleontology, geology, genetics, phylogenetic methods, and functional morphology, among others.

Every extinct species is a window to the past

Sergio Almecija

“Every extinct species is a window to the past. Chimpanzees and humans share a common ancestor that lived towards the end of the Miocene. To infer what this last common ancestor between apes and humans was like, it is essential to understand what the apes that lived before the divergence were like ”, Spanish paleontologist Sergio Almecija, from the American Museum of Natural History, who is leading the research, tells SINC. The Catalan Institute of Paleontology Miquel Crusafont and the New York Institute of Technology (USA) also participate in the study.

Deciphering the origin of our lineage

There are two main approaches to solving the problem of human origins: the “descending”, which is based on the analysis of living apes, especially chimpanzees; and the “ascendant”, which gives importance to the largest tree of the apes, most of which are extinct.

In this way, some scientists assume that hominids arose from an ancestor that walked with the knuckles, similar to chimpanzees. Others argue that it was from an ancestor more similar, in some features, to part of the strange apes of the Miocene.

“Darwin speculated that humans originated in Africa from an ancestor different from any living species. However, he remained cautious given the scarcity of fossils at the time,” Almécija explains.

Sergio Almécija at the AMNH. / Matt Shanley

The researchers in this paper explain that top-down studies sometimes ignore the reality that living apes – humans, chimpanzees, gorillas, orangutans, and hylobatids – are just the survivors of a much larger and now extinct group. On the other hand, those based on the bottom-up approach are likely to give individual fossil apes an important evolutionary role.

A species with unique features

Humans split from apes – specifically, the chimpanzee lineage – sometime between 9.3 and 6.5 million years ago, towards the end of the Miocene. To understand the origins of hominids, the paleoanthropologists they have tried to reconstruct the physical characteristics, behavior and environment of the last common ancestor of humans and chimpanzees.

Miocene fossil ape skeletons show combinations of features that do not exist today

The group of hominoids Living organisms include humans and what we commonly call apes. A notable characteristic of all of them is our orthograde body shape, that is, erect. “For example, both chimpanzees and humans have a wide, shallow rib cage and a short, stiff lower back. Instead, today’s monkeys have a quadruped body plan. Like most of the mammals –Dogs and cats, for example– have a narrow and deep torso, with a long and flexible spine in the lower back ”, continues Almecija.

Fossil ape skeletons from the Miocene show combinations of features that do not exist today. The so-called Sivapithecus (12 to 9 million years old, found in India and Pakistan) shows an orangutan-like face with a body more similar to a quadruped monkey.

For its part, Pierolapithecus (12 million years old, Spain) shows a body plan and size similar to a current great ape. However, its vertebrae were similar to those of a gibbon and his hands shorter than those of a chimpanzee. “These strange combinations are the root of the problem. There is no consensus among researchers on how to interpret these strange apes from the Miocene”, Says the expert.

To assume that our ancestor with the chimpanzee was no different from a current chimpanzee is to deny that the latter have evolved

Sergio Almecija

These unique characteristics are due to the fact that each species exhibits its unique mosaic of primitive and derived characters. For example, humans have five fingers on each hand and foot, a primitive feature present in almost all primates and mammals.

“Our slow development is not extremely different from that of the great apes current (orangutans, gorillas and chimpanzees). Therefore, it is a primitive characteristic for primates, but derived for hominids. At the same time, we show a uniquely derived type of locomotion, among primates: bipedalism usual terrestrial ”, adds the researcher.

The first fossils thought to be in the human lineage, represented by various parts of the body, show combinations of characteristics. Some of them are present in modern apes. Others are only represented in Miocene apes. For example, Orrorin tugenensis (6 million years old, Kenya) has a thumb and femur similar to that of Australopithecus afarensis (“Lucy”, around 3 million years old). However, other features of the femur are more similar to a Kenyan ape from 18 million years ago than to a current chimpanzee.

“To assume that our ancestor with the chimpanzee was not different from a current chimpanzee is to deny that the latter have evolved,” concludes Almecija.

The variety of positions that precedes human bipedalism is unknown. / © Sergio Almécija

Reference: S. Almécija et al. “Fossil apes and human evolution.” Science