Ancient Hobbits slowed down growth during childhood, showing that humans didn't always grow 'bigger and bigger brains'

Until Homo floresiensis was discovered, scientists assumed that the evolution of the human lineage was defined by bigger and bigger brains. Via a process called encephalization, human brains evolved to be relatively more massive than would be expected based on corresponding body size.

This proportionally bigger brain is what anthropologists argued enabled us and our relatives to perform more complex tasks such as using fire, forging and wielding tools, making art and domesticating animals.

But these theories had to be thrown out the window when archaeologists announced our fossil cousins Homo floresiensis via scientific publication in 2004. Homo floresiensis lived from about 700,000 to 60,000 years ago in the rainforests of Indonesia, partially contemporaneous with our own species.

Aptly nicknamed Hobbits, Homo floresiensis were short-statured, at just over 3 feet (1 meter) tall, and had a chimp-size brain. This discovery upended the assumption that brains have been increasing in size over the past several million years and generated confusion about what separates recent human relatives in our genus Homo from our more ancient ancestors.

Our new research on the skulls and teeth provides a novel theory for how the Hobbits evolved to be small.

We are professors of anthropology at Western Washington University. After attending a 2023 workshop for biological anthropologists studying juveniles in the fossil record, we began looking at brain size changes across human evolution.

Related: Archaic human 'hobbits' were even shorter than we thought, 700,000-year-old teeth and bone reveal

Our previous work on the proportions of molar teeth generated new insights into the evolution of pregnancy by demonstrating that fetal growth rates are tightly linked to molar proportions in primates. Now, we wanted to see whether we could uncover a relationship between tooth proportions and brain size among our fossil relatives.

Paleontologists have only limited skeletal materials, sometimes only a few teeth, for many fossil species, including Homo floresiensis. If tooth proportions can provide information about fossil brain size, it opens up a world of possibilities for assessing past changes in encephalization.

Reconstructing brain size using teeth

We collated data on tooth and brain size for 15 fossil species on the human family tree, spanning about 5 million years of evolution. Somewhat oxymoronically, the third molars — otherwise known as wisdom teeth — have gotten proportionally smaller as brain size has gotten larger throughout human evolution, for most species.

Overall, human relatives with relatively larger wisdom teeth are more ancient and had smaller brains. More recent taxa, like Homo neanderthalensis, had relatively smaller third molars, compared to their other teeth, and larger brains.

This relationship allows researchers to figure out something about brain size for fossils that are incomplete, perhaps existing only as a few lone teeth. Since teeth are predominately made of inorganic matter, they survive in the fossil record much more often than other parts of the body, making up the vast majority of paleontological materials recovered. Being able to know more about brain size from just a few teeth is a truly useful tool.

Scientists recognize now that the formation of the brain and the teeth are inextricably connected during gestation. And for most species, larger brains are correlated with smaller wisdom teeth.

The one exception in genus Homo is Homo floresiensis, the Hobbit. The wisdom teeth of the Hobbits are small proportional to the other molars — the typical pattern for members of genus Homo. But their brains are also small, which is quite unusual.

There are two primary ways for brain size to decrease: by slowing down growth during gestation before birth or by slowing down growth after birth, during childhood. Because teeth develop early in gestation, slowing down growth rates during pregnancy tends to affect tooth shape and size, or even whether the teeth develop at all. Slowing growth later, during childhood, influences skeletal shape and size in other ways, because different parts of the body develop at different times.

Our new research provides evidence that the body size of Homo floresiensis likely shrank from a larger-bodied Homo ancestor by slowing down growth during childhood. The Hobbits' small wisdom teeth suggest that, at least in utero, they were on track for the proportionally bigger brains that are the trademark of humans and their relatives. Any brake that slowed down brain growth likely occurred after birth.

In fact, this is the same mechanism through which some short-statured modern human populations have adapted to their local ecological conditions.

Getting small on islands

The small body size of Homo floresiensis was likely an adaptation to the unique conditions of their island environment on Flores.

Evolving small body size as an adaptation to living on an isolated island is known as insular nanism. There are many examples of other mammals becoming small on islands over the past 60 million years. But one of the most relevant examples is the dwarf elephant, Stegodon sondaarii, that lived on Flores and was hunted by H. floresiensis for food.

Both Homo floresiensis and Homo luzonensis, another short, island hominin from Southeast Asia, likely evolved very short stature because of the ecological effects of limited food availability and lack of large predators, which tends to characterize island habitats.

Because brain size and body size are tightly linked, body size evolution inherently affects brain evolution. Among modern humans, larger people have larger brains, and smaller people have smaller brains.

But people with smaller brains are certainly no less intelligent than people with larger brains. Variation in body size dictates brain size; it is not a measure of cognitive ability. The island Hobbits crafted tools, hunted large-for-them game in the form of pygmy elephants, and likely made and used fire.

Our research supports that their small body size originated from a slowdown in growth during childhood. But this process would likely have had little impact on brain function or cognitive ability. We hypothesize that the Hobbits were small but highly capable.

Understanding the evolution of us

New research, including our study, continues to reinforce the importance of understanding how pregnancy and child growth and development evolved. If we want to know what distinguishes humans from our evolutionary ancestors, and how we evolved, we must understand how the earliest moments of life have changed and why.

Our work also encourages the reevaluation of endless attention on increasing brain size as the predominant force in human evolution. Other species in genus Homo had small brains but were likely not much different from us.

This edited article is republished from The Conversation under a Creative Commons license. Read the original article.