Illustration by Christopher M. Smith
One hundred fifty years ago, Charles Darwin speculated that humans originated in Africa. Since Darwin’s time, the number of species in the human family tree has grown exponentially, but little is known about the common ancestor between living apes and humans or why these branches diverged.
Fossil evidence from this area of the primate family tree is scarce, consisting mainly of singular teeth and remains of broken jawbones. The limited evidence makes it challenging for scientists to imagine what the last common ancestor might have looked like.
While most of these paleoanthropologists share a common goal of solving the human origin puzzle, many are also at odds as to whether some fossils play a significant role, with fossil apes at the center of these debates. Some scientists believe that fossil ape remains are the remnants of critical species that played a starring role in the human origin story while others dismiss these theories and prefer to focus instead on the connection between humans and living apes. However, this lack of consensus could stall necessary progress to truly discovering why the human species emerged.
A new report published May 7 in the journal Science may help to form that consensus and demonstrate why the role of fossil apes should not be discounted. Nathan Thompson, Ph.D., assistant professor of anatomy at the College of Osteopathic Medicine, joined researchers from the American Museum of Natural History and the Catalan Institute of Paleontology for the study, which presents a well-documented case on fossil apes’ ability to provide insight on ape and human evolution, including information on our last common ancestor. (Watch a video about the research.)
Early humans (hominins) diverged from apes, specifically chimpanzees, somewhere between 9.3 million and 6.5 million years ago during the Miocene period. To understand hominin origins, paleoanthropologists often try to reconstruct the physical characteristics, behavior, and environment of the last common ancestor shared by both humans and chimps. These scientists generally fall into one of two camps: those that use a “top-down” approach vs. those that use a “bottom-up” approach. Those who use a top-down approach work backward in time, using humans as a starting point, and believe that living apes provide the most significant clues on the human lineage. Those who use a bottom-up approach tend to rely on the earliest known ancestors as their starting point, working from past to present, and prioritize the larger tree of mostly extinct ape ancestors.
Thompson and his co-authors—which include experts with backgrounds ranging from paleontology to functional morphology, and phylogenetics—discuss the limitations of relying exclusively on one of these two opposing approaches. They argue that a top-down approach might ignore the reality that living apes are simply the descendants of a much larger, and now mostly extinct, group. On the other hand, they note that a bottom-up approach might make scientists more inclined to side with their own biases, causing them to assign an important evolutionary role to individual fossil apes in order to fit their desired narrative.
While the study of science is intended to be objective, those performing the research are human, and humans are natural storytellers. There is an innate desire to understand where one’s species came from, but overreaching or trying to force a fossil into pre-conceived ideas about human evolution can inadvertently cloud the objective scientific process, which relies on hypotheses and methodical testing. They write:
“Theories of human evolution often resemble ‘anthropogenic narratives’ that borrow the structure of a hero’s journey to explain essential aspects such as the origins of erect posture, the freeing of the hands, or brain enlargement. Intriguingly, such narratives have not drastically changed since Darwin.”
They go on to note that researchers must be aware of confirmation biases and ad hoc interpretations, as some scientists may try to cast their new fossil with the starring role in a preexisting narrative or the limited amount of knowledge available today. Linking a new fossil to evolutionary scenarios is appealing because these ideas provide intriguing and possible explanations based on current knowledge, “but unless grounded in testable hypotheses, they are no more than ‘just-so stories,” the authors note.
Even making claims about the “birthplace” of early humans can be complicated. In his 1871 work The Descent of Man, Darwin speculated that humans originated in Africa from an ancestor that was completely different from any living species, but he remained cautious about definitively making this claim because fossil evidence was scarce in the 1800s. Today, the picture is still unclear, but the discovery of more fossils has led most scientists to believe that remains found in eastern and central Africa could be those of hominins that lived around the time of the human-chimpanzee divergence. Meanwhile, other scientists contend that fossils found in Europe indicate that humans originated there.
Today, more than 50 fossil ape species are documented across Africa and Eurasia. However, these fossil apes have an assortment of features that are not in line with the features found on today’s apes. Living apes share common features related to their movement (locomotion), including broad torsos with widely spaced shoulders, stiff backs, long forelimbs, mobile limb joints, strong grasping ability, and the absence of a tail. However, many of these traits are often missing in the earliest fossil ape forms. These contradicting features are yet another reason why scientists are challenged with forming a consensus.
The Importance of Fossil Apes
After reviewing the existing scientific literature, the researchers found that most human origin stories, whether they are based on a top-down or bottom-approach approach, are simply not compatible with the fossils available today. Given this, scientists need to re-examine these stories and the way that they approach their studies.
The authors also posit that today’s living apes consist of specialized species, which are the descendants of a much larger tree of extinct ape ancestors, carrying specific traits passed down from their ancestors. Their larger, complex family tree suggests that a human evolutionary story based on today’s few living ape species would be missing the bigger picture.
“When you consider all of the evolutionary research that’s been conducted in the last 150 years, to say that humans simply originated from today’s living ape species glosses over so much of our larger evolutionary history,” says Thompson, whose previous work has shed light on bipedalism and how humans came to walk on two feet.
While piecing together the human evolution puzzle is challenging, the authors note that current disagreements regarding ape and human evolution would be much more informed if, together with early hominins and living apes, Miocene apes were part of the equation.
“Fossil apes have a very unique assortment of features, which are often very different from the features we find in today’s apes,” says Thompson. “Understanding these variations, and what they meant for how our fossil apes lived and interacted with their environment, is crucial for understanding which human traits and behaviors are truly unique.”