The great apes, including humans, gorillas, chimpanzees, bonobos and orangutans, are very intelligent.

The gorilla Koko (who was born at the San Francisco Zoo and lived in California) was taught to communicate with her teacher, Francine Patterson. The chimpanzee Washoe (born in West Africa in 1965 and captured for use by the US Air Force for research on the US space programme) was taught to use sign language and learned about 350 signs. Kanzi, (born at Yerkes Field Station at Emory University, Atlanta, and moved to the Language Research Center at Georgia State University), a bonobo, not only developed good English comprehension and syntax, but also made stone tools. 

Skulls of great ape species.

But how does the intelligence of living great apes, such as Koko the gorilla, compare with our 3 million-year-old relatives, such as “Lucy” the Australopithecine?  Lucy (a 40% complete female specimen of the hominin species Australopithecus afarensis discovered in 1974 in Ethiopia) is generally considered to have been smarter because the size of her brain, indicated by the fossil braincases of Australopithecus species, is larger than orangutans and chimpanzees, and is comparable to gorillas despite the great difference in body mass.

However, a study published this week in the prestigious Proceedings of the Royal Society B biological research journal, challenges this idea. Research led by Professor Roger Seymour at the University of Adelaide, in collaboration with Dr Edward Snelling from the University of Pretoria’s Department of Anatomy and Physiology in the Faculty of Veterinary Science; Prince Chikezie of the Brain Function Research Group, Faculty of Health Sciences at the University of the Witwatersrand (Wits); and Dr Bernhard Zipfel (Evolutionary Studies Institute at Wits), reveals a significantly higher rate of blood flow to the cognitive part of the brain of living great apes compared to Australopithecus.

Previously, scientists assumed there was no way to measure the blood flow requirements or metabolic rate of an organ, such as the brain, in an animal that has been dead for millions of years. This was until Prof Seymour realised that if one has access to fossil skulls that contain the carotid foramina intact, then one could measure the size of the foramina holes, then calculate the size of the vessels that would have gone through the holes. “If you know the size of the vessel, then you can calculate the blood flow along that vessel as it would have been in the animal when it was alive,” said Prof Seymour.

Emeritus Professor Roger Seymour of the University of Adelaide.

As a comparative cardiovascular physiologist, he came up with a simple idea on how to measure blood flow rate by the size of the holes in fossil skulls. “It was much more difficult to validate the idea and gather the data on human ancestors, but with key collaborations in South Africa, it became reality,” he said. The new research measures the rate of blood flow to the cognitive part of the brain, based on the size of the holes in the skull that allow passage for the supply arteries. This technique was calibrated in humans and other mammals, and then applied to 96 great ape skulls and 11 Australopithecus fossil skulls. The study shows that cerebral blood flow rate in our australopithecine human ancestors was well below that of living (non-human) great apes.

According to Dr Snelling, fossil specimens were measured from museums and academic institutions in the US, Australia, Wits University’s School of Anatomical Sciences and Evolutionary Studies Institute, and the Ditsong National Museum of Natural History in Pretoria. “It took several years to acquire all the data, run their calculations, calibrate and understand and interpret the results.” He led the data analysis.

He said in many specimens of hominins (our recent human ancestors) that have been unearthed, “we have been able to take measurements of the carotid foramina, to then calculate the internal carotid artery vessel size, and then calculate blood flow rate to the cognitive part of the brain (the cerebrum).  We then compared our calculations for Australopithecus to the gorillas, orangutans, and chimps. What we found surprised us – these apes have significantly higher brain blood flow rates than Australopithecus. Blood flow to most organs is related to the metabolic rate of the organ.” 

For Dr Snelling, a brain with a higher metabolic rate suggests a higher level of cognition and intelligence because being smart is energetically costly. “So it seems the brain blood flow rate, and hence brain metabolic rate, and perhaps cognition, is much higher in living great apes compared to what it was in Australopithecus. The reason this is surprising is because we had previously assumed, based on brain size alone, that our three-million-year-old ancestors were at least as cognitively advanced as the living (non-human) great apes, but these results cast serious doubt on that assumption.”

Dr Edward Snelling from the University of Pretoria’s Department of Anatomy and Physiology in the Faculty of Veterinary Science.

The blood flow rates that they calculated were much lower in the australopithecines. “For example, Australopithecus and the gorilla have a brain volume of about 450 mL, but brain blood flow is nearly twice as high in the gorilla compared to our 3-million-year-old ancestor.”

He said that the tight link between blood flow rate and metabolic rate suggests that Australopithecus did not have as high a brain metabolic rate as assumed, based on brain size alone. “It might even suggest that their level of cognition was significantly inferior to that of the gorilla and indeed the other non-human great apes.  This is no criticism of Australopithecus – remember that gorillas are extremely smart primates!”

It has generally been assumed that intelligence is directly related to the size of the brain.  At first, brain size seems reasonable, because it is a measure of the number of brain cells, called neurons. However, on second thought, cognition relies not only on the number of neurons, but also on the number of connections between them, called synapses.  These connections govern the flow of information within the brain, and greater synaptic activity results in greater information processing.

According to Dr Snelling, the human evolutionary lineage separated from those of the other great apes about 6 to 10 million years ago, and brain size tended to increase within all lineages.  It appears that brain size and blood flow rate started out low but increased more quickly in the human lineage than in the other great ape lineages.

Based on the results, it is estimated that blood flow to Koko’s cerebral hemispheres was about twice that of Lucy’s, despite having similarly sized brains. Because blood flow rate might be a better measure of information processing capacity than brain size alone, Koko the gorilla seems to have been smarter than Lucy the Australopithecine. “It matters because Australopithecus is the ‘recent’ ancestor to all humans, and we previously assumed that they must have had a fairly high level of cognition (based on brain size being comparable to modern [non-human] great apes), but what we show is that the level of cognition (inferred from brain blood flow calculations) in Australopithecus was probably not as high as we think, and not as high as that in the living great apes (gorillas, chimps, orangutans). It thus changes our perspective on who we think we are as a species and how we evolved our intelligence.”

The findings of this study modify scientists’ understanding of the evolution of human cognition and intelligence.