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New research suggests that humans became the large-brained, large-bodied animals we are today because of natural selection to increase brain size. The work, published in the journal Current Anthropology, contradicts previous models that treat brain size and body size as independent traits responding to separate evolutionary pressures. Instead, the study shows that brain size and body size are genetically linked and that selection to increase brain size will "pull along" body size. This phenomenon played a large role in both brain- and body-size increases throughout human evolution and may have been solely responsible for the large increase in both traits that occurred near the origins of our genus, Homo.
Over the last four million years, brain size and body size increased substantially in our human ancestors," said paper author Mark Grabowski, a James Arthur postdoctoral fellow in the Division of Anthropology at the American Museum of Natural History. "This observation has led to numerous hypotheses attempting to explain why observed changes occurred, but these typically make the assumption that brain- and body-size evolution are the products of separate natural selection forces.
A ‘Major Milestones’ in Human Evolution Diorama in Kolkata, India. ( Biswarup Ganguly/ CC BY 3.0 )
That assumption is now being questioned, based on a large body of work that has shown that genetic variation--the fuel of evolution--in some traits is due to genes that also cause variation in other traits, with the result that selection on either trait leads to a correlated response in the unselected trait. Consider the leg bone, or femur, of an elephant. As the bone gets longer, it also gets wider. If artificial selection is used to produce a tall elephant, its legs likely won't just become long, they'll also get wider. Part of this effect is due to shared genetic variation, or covariation, among traits in the femur. Grabowski set out to explore this kind of genetic relationship between human brain size and body size, and its impact on our evolution.
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With brain- and body-size covariation patterns from a range of primates and modern humans, Grabowski created a number of models to examine how underlying genetic relationships and selection pressures likely interacted across the evolution of our lineage.
Left: Reconstruction of "Lucy" (an Australopithecus) using its remains. Natural History Museum, Washington DC, USA ( CC BY SA 3.0 ) Right: A model of a face of an adult female Homo erectus, one of the first truly human ancestors of modern humans, on display in the Hall of Human Origins in the Smithsonian Museum of Natural History in Washington, D.C. ( CC BY SA 2.0 )
His findings demonstrate, for the first time, that strong selection to increase brain size alone played a large role in both brain- and body-size increases throughout human evolution. This phenomenon also may have been solely responsible for the major increase in both traits that occurred during the transition from human ancestors like Australopithecus (the most famous of which is the Lucy fossil) to Homo erectus.
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Australopithecus afarensis compared to Homo erectus. ( Laszlo Meszoly, Harvard U .)
In other words, while there are many scientific ideas explaining why it would be beneficial for humans to evolve bigger bodies over time, the new work suggests that those hypotheses may be unnecessary; instead, body size just gets pulled along as the brain expands.
While selection no doubt played a role in refining the physical changes that came with larger body sizes, my findings suggest it was not the driving force behind body-size evolution in our lineage," Grabowski said. "Therefore, evolutionary models for the origins of Homo based on an adaptive increase in body size need to be reconsidered.
Homo erectus from Tautavel, France (reconstruction, Museum Tautavel). ( CC BY SA 3.0 )
Featured image: One Million Years of the Human Story at the Natural History Museum. Source: ( CC BY 2.0 )
The article ‘ Bigger brains led to bigger bodies in our ancestors ’ was originally published on Science Daily .
Source: American Museum of Natural History . "Bigger brains led to bigger bodies in our ancestors: Study examines correlated evolution of brain, body size in humans." ScienceDaily. ScienceDaily, 18 April 2016.
When Fire Met Food, The Brains Of Early Humans Grew Bigger
If you're reading this blog, you're probably into food. Perhaps you're even one of those people whose world revolves around your Viking stove and who believes that cooking defines us as civilized creatures.
Actors Stan Laurel and Edna Marlon play at socializing around the campfire. It turns out that early man's brain developed in part thanks to cooking. Hulton Archive/Getty hide caption
Actors Stan Laurel and Edna Marlon play at socializing around the campfire. It turns out that early man's brain developed in part thanks to cooking.
Well, on the latter part, you'd be right. At least according to some neuroscientists from Brazil.
They noticed (haven't we all?) that humans have very big brains. But they point out that gorillas and orangutans have bigger bodies than we do by far, but smaller brains and fewer neurons. Normally, brain size pretty much matches body size in primates. So how did we get so well-endowed?
Big bodies need a lot of energy. And the brain is especially a real calorie hog: About 20 percent of what we consume goes to the brain, even though it's only 2 percent of our body mass.
Our primate ancestors had to graze almost constantly to get enough calories from stuff like raw tubers or other vegetable matter. There was raw meat now and then, but by and large the apes, and our ancestors, were about as neurologically developed as their diets would allow, as we've reported before.
If you wanted a bigger brain, you had to downsize the rest of your body. In fact, the Brazilian scientists calculated that for a gorilla to get enough extra energy to grow a brain as big as ours, it would have to eat another two hours a day, on top of the nine hours or so it already spends feeding.
So what happened was, early humans discovered cooking. If you believe the British writer Charles Lamb, raw meat-eating humans kept pigs in their houses until one day a house burned down, pig inside, and we discovered roast pork.
The Brazilian scientists, however, don't speculate on how we stumbled on cooking (though Brazilians have earned a worthy reputation for refining the art of barbecuing, which they call churrasco).
What the scientists conclude is that cooking made food easier to chew and digest. Moreover, cooking releases more calories to the body in some cases. And it makes old meat that a dog wouldn't eat go down a little easier. The same benefits of cooking go for tubers and veggies, too.
Oh, and don't overlook the fact that spending less time grazing and more time gathered around the fire gave us more opportunity to schmooze, which also may have helped hone our brains. If you don't believe any of this, you can check out the research by certified brain scientists, in the Proceedings of the National Academy of Sciences. In fact, there's quite a body of scientific literature out there that backs up this idea, which has long been championed by famous Harvard primatologist (and vegetarian) Richard Wrangham.
So raise a glass of good wine (fermentation being the other calling card of a higher order brain) and praise the cooks. You'd be stupid without them.
Bigger brains led to bigger bodies in our ancestors
Study examines the correlated evolution of brain and body size in humans.
AMERICAN MUSEUM OF NATURAL HISTORY—New research suggests that humans became the large-brained, large-bodied animals we are today because of natural selection to increase brain size. The work, published in the journal Current Anthropology, contradicts previous models that treat brain size and body size as independent traits responding to separate evolutionary pressures. Instead, the study shows that brain size and body size are genetically linked and that selection to increase brain size will “pull along” body size. This phenomenon played a large role in both brain- and body-size increases throughout human evolution and may have been solely responsible for the large increase in both traits that occurred near the origins of our genus, Homo.
“Over the last four million years, brain size and body size increased substantially in our human ancestors,” said paper author Mark Grabowski, a James Arthur postdoctoral fellow in the Division of Anthropology at the American Museum of Natural History. “This observation has led to numerous hypotheses attempting to explain why observed changes occurred, but these typically make the assumption that brain- and body-size evolution are the products of separate natural selection forces.”
That assumption is now being questioned, based on a large body of work that has shown that genetic variation—the fuel of evolution—in some traits is due to genes that also cause variation in other traits, with the result that selection on either trait leads to a correlated response in the unselected trait. Consider the leg bone, or femur, of an elephant. As the bone gets longer, it also gets wider. If artificial selection is used to produce a tall elephant, its legs likely won’t just become long, they’ll also get wider. Part of this effect is due to shared genetic variation, or covariation, among traits in the femur. Grabowski set out to explore this kind of genetic relationship between human brain size and body size, and its impact on our evolution.
New research shows that a strong selection to increase brain size alone played a large role in both brain- and body-size increases throughout human evolution. This phenomenon also may have been solely responsible for the major increase in both traits that occurred during the transition from human ancestors like Australopithecus, a model of which is seen here in the American Museum of Natural History’s Hall of Human Origins, to Homo erectus. Credit: AMNH/R. Mickens
With brain- and body-size covariation patterns from a range of primates and modern humans, Grabowski created a number of models to examine how underlying genetic relationships and selection pressures likely interacted across the evolution of our lineage. His findings demonstrate, for the first time, that strong selection to increase brain size alone played a large role in both brain- and body-size increases throughout human evolution. This phenomenon also may have been solely responsible for the major increase in both traits that occurred during the transition from human ancestors like Australopithecus (the most famous of which is the Lucy fossil) to Homo erectus.
In other words, while there are many scientific ideas explaining why it would be beneficial for humans to evolve bigger bodies over time, the new work suggests that those hypotheses may be unnecessary instead, body size just gets pulled along as the brain expands.
“While selection no doubt played a role in refining the physical changes that came with larger body sizes, my findings suggest it was not the driving force behind body-size evolution in our lineage,” Grabowski said. “Therefore, evolutionary models for the origins of Homo based on an adaptive increase in body size need to be reconsidered.”
Blustery days and bigger brains
As our closest ancestors evolved, brain size began to increase substantially in members of our own genus, Homo. Scientists believe that there is a link between brain size and how variable the climate is. The logic is that living in a variable climate makes life unpredictable. When climate is unpredictable, food and water are more difficult to find. It may take more time and effort to find them. Having a large brain allows animals to think their way through these unpredictable conditions.
About two million years ago, the climate began to be more variable than it had been before that. This variability in climate coincided with brain size increases in early Homo. Bigger brains allowed early Homo species to survive in variable environments. For example, bigger brains may have allowed Homo to be a more strategic hunter or scavenger.
The bodies of early Homo were also more human-like than those of early hominins. This may have allowed early Homo to travel and run for long distances, perhaps even running their prey to exhaustion. Early Homo also had more complex stone tools. Such tools would have made killing and butchering animals easier.
The trend of climatic instability continued from 800,000 to 200,000 years ago. The greatest variation in climate in all of human history occurred during this time. This also coincided with the greatest increases in brain size, and brains eventually reached the size they are in modern humans. One late Homo species, the Neanderthal, grew brains that even exceeded modern human brain size. Neanderthals are associated with complex stone tools, and they were excellent hunters.
Bigger Brains Drove Evolution to Bigger Human Bodies
New research suggests that humans became the large-brained, large-bodied animals we are today because of natural selection to increase brain size.
The work, published in the journal Current Anthropology, contradicts previous models that treat brain size and body size as independent traits responding to separate evolutionary pressures.
Instead, the study shows for the first time that brain size and body size are genetically linked, and that selection to increase brain size will “pull along” body size, a phenomenon that may have played a key role in the increase in both traits that occurred near the origins of modern humans and other species in the genus Homo.
“Over the last four million years, brain size and body size increased substantially in our human ancestors,” said paper author Mark Grabowski, a James Arthur postdoctoral fellow in the Museum's Division of Anthropology. “This observation has led to numerous hypotheses attempting to explain why observed changes occurred, but these typically make the assumption that brain- and body-size evolution are the products of separate natural selection forces.”
In the study, Grabowski created a number of models to examine how underlying genetic relationships and selection pressures likely interacted across the evolution of our lineage. His findings demonstrate for the first time that strong selection to increase brain size alone impacted both brain- and body-size increases throughout the course of human evolution and demonstrated in fossils of species like Australopithecus and Homo erectus.
While there are many scientific ideas explaining why it would be beneficial for humans to evolve bigger bodies over time, the new work suggests that those hypotheses may be unnecessary instead, body size just gets pulled along as the brain expands.
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People talk a lot about intelligence and brain size. Something that’s most always brought up is how the human brain increased in size the past 4 million years. According to PP, the trend for bigger brains in hominins is proof that evolution is “progressive”. However, people never talk about a major event in human history that caused our brains to suddenly increase: the advent of fire. When our ancestors mastered fire, it was then possible for the brain to get important nutrients that influenced growth. People say that “Intelligence is the precursor to tools”, but what if fire itself is the main cause for the increase in brain size in hominins the past 4 million or so years? If this is the case, then fire is, in effect, the ultimate cause of everything that occurred after its use.
The human brain consumes 20-25 percent of our daily caloric intake. How could such a metabolically expensive organ have evolved? The first hominin to master fire was H. erectus. There is evidence of this occurring 1-1.5 mya. Not coincidentally, brain size began to tick upward after the advent of fire by H. erectus. Erectus was now able to consume more kcal, which in turn led to a bigger brain and the beginnings of a decrease in body size. The mastery and use of fire drove our evolution as a species, keeping us warm and allowing us to cook our food, which made eating and digestion easier. Erectus’s ability to use fire allowed for the biggest, in my opinion, most important event in human history: cooking.
With control of fire, Erectus could now cook its foods. Along with pulverizing plants, it was possible for erectus to get better nutrition by ‘pre-digesting’ the food outside of the body so it’s easier to digest. The advent of cooking allowed for a bigger brain and with it, more neurons to power the brain and the body. However, looking at other primates you see that they either have brains that are bigger than their bodies, or bodies that are bigger than their brains, why is this? One reason: there is a trade-off between brain size and body size and the type of diet the primate consumes. Thinking about this from an evolutionary perspective along with what differing primates eat and how they prepare (if they do) their food will show whether or not they have big brains or big bodies. How big an organism’s brain gets is directly correlated with the amount and quality of the energy consumed.
There is a metabolic limitation that results from the number of hours available to feed and the low caloric yield of raw foods which then impose a trade-off between the body size and number of neurons which explains why great apes have small brains in comparison to their bodies. Metabolically speaking, a body can only handle one or the other: a big brain or a big body. This metabolic disadvantage is why great apes did increase their brain size, because their raw-food diet is not enough, nutritionally speaking, to cause an increase in brain size (Azevedo and Herculano-Houzel, 2016). Can you imagine spending what amounts to one work day eating just to power the brain you currently have? I can’t.
Energy availability and quality dictates brain size. A brain can only reach maximum size if adequate kcal and nutrients are available for it.
Total brain metabolism scales linearly with the number of neurons (Herculano-Houzel, 2011). The absolute number of neurons, not brain size, dictates a “metabolic constraint on human evolution”, since people with more neurons need to sustain them, which calls for eating more kcal. Mammals with more neurons need to eat more kcal per day just to power those brains. For instance, the human brain needs 519 kcal to run, which comes out to 6 kcal per neuron. The brain is hugely metabolically expensive, and only the highest quality nutrients can sustain such an organ. The advent of fire and along with it cooking is one of, if not the most important reason why our brains are large (compared to our bodies) and why we have so many neurons compared to other species. It allowed us to power the neurons we have, 86 billion in all (with 16 billion in the cerebral cortex which is why we are more intelligent than other animals, number of neurons, of course being lower for our ancestors) which power human thought.
The Expensive Tissue Hypothesis (ETA) explains the metabolic trade-off between brain and gut, showing that the stomach is dependent on body size as well as the quality of the diet (Aiello, 1996). As noted above, there is good evidence that erectus began cooking, which coincides with the increase in brain size. As Man began to consume meat around 1.5 million years ago, this allowed for the gut to get smaller in response. If you think about it, it makes sense. A large stomach would be needed if you’re eating a plant-based diet, but as a species begins to eat meat, they don’t need to eat as much to get the adequate amount of kcal to fuel bodily functions. This lead to the stomach getting smaller, and along with it so did our jaws.
So brain tissue is metabolically expensive but there is no significant correlation between brain size and BMR in humans or any other encephalized mammal, the metabolic requirements of relatively large brains are offset by a corresponding gut reduction (Aiello and Wheeler, 1995). This is the cause for the low, insignificant correlation between BMR and our (relatively large brains, which correlates to the amount of neurons we have since our brains are just linearly scaled-up primate brains).
Evidence for the ETA can be seen in nature as well. Tsuboi et al (2015) tested the hypothesis in the cichlid fished of Lake Victoria. After they controlled for the effect of shared ancestry and other ecological variables, they noted that brain size was inversely correlated with gut size. Perhaps more interestingly, they also noticed that when the fish’s’ brain size increased, increased investment and paternal care occurred. Moreover, more evidence for the ETA was found by Liao et al (2015) who found a negative correlation between brain mass and the length of the digestive tract within 30 species of Anurans. They also found, just like Tsuboi et al (2015), that brain size increase accompanied an increase in female reproductive investment into egg size.
Moreover, another cause for the increase in brain size is our jaw size decreasing. This mutation occurred around 2.4 million years ago, right around the time frame that erectus discovered fire and began cooking. This is also consistent with, of course, the rapid increase in brain size which was occurring around that time. The room has to come from somewhere, and with the advent of cooking and meat eating, the jaw was, therefore, able to get smaller along with the stomach which increased brain size due to the trade-off between gut size and brain size. Morphological changes occurred exactly at the same time changes in brain size occurred which coincides with the advent of fire, cooking, and meat eating. Coincidence? I think the evidence strongly points that this is the case, the rapid increase in brain size was driven by fire, cooking, and meat eating.
The rise of bipedalism also coincided with the brain size increase and nutritional changes. Bipedalism freed the hands so tools could be made and used which eventually led to the control of fire. Lending more credence to the hypothesis of bipedalism/tools/brain size is the fact that there is evidence that the first signs of bipedalism occurred in Lucy, our Australopithecine ancestor who had pelvic architecture that showed she was clearly on the way to bipedalism. There is more evidence for bipedalism in fossilized footprints of australopithecines around 3 mya, coinciding with Lucy, tool use and eventually the advent and use of fire as a tool to cook and ward off predators. Ancient hominids could then better protect their kin, have higher quality food to eat and use the fire to scare off predators with.
The nutritional aspect of evolution and how it co-evolved with us driving our evolution in brain size which eventually led to us is extremely interesting. Without proper nutrients, it’s not metabolically viable to have such a large brain, as whatever kcal you do eat will need to go towards other bodily functions. Moreover, diet quality is highly correlated with brain size. Great apes can never get to the brain size that we humans have, and their diet is the main cause. The discovery and control of fire, the advent of cooking and then meat eating was what mainly drove the rapid increase of brain size starting 4 mya.
In a way, you can think of the passing down of the skill of fire-making to kin as one of the first acts of cultural transference to kin. It’s one of the first means of Lamarckian cultural transference in our history. Useful skills for survival will get passed down to the next generation, and fire is arguably the most useful skill we’ve ever come across since it’s had so many future implications for our evolution. The ability to create and control fire is one of the most important skills as it can ward off predators, cook meat, be used to keep warm, etc. When you think about how much time was freed up upon the advent of cooking, you can see the huge effect the control of fire first had for our species. Then think about how we could only control fire if our hands were freed. Then human evolution begins to make a lot more sense when put into this point of view.
When thinking about brain size evolution as well as the rapid expansion of brain size evolution, nutrition should be right up there with it. People may talk about things like the cold winter hypothesis and intelligence ad nauseam (which I don’t doubt plays a part, but I believe other factors are more important), but meat-eating along with a low waist-to-hip ratio, which bipedalism is needed for all are much more interesting when talking about the evolution of brain size than cold winters. All of this wouldn’t be possible without bipedalism, without it, we’d still be monkey-like eating plant-based diets. We’d have bigger bodies but smaller brains due to the metabolic cost of the plant-based diet since we wouldn’t have fire to cook and tools to use as we would have still been quadrupeds. The evolution of hominin intelligence is much more interesting from a musculoskeletal, physiological and nutritional point of view than any simplistic cold winter theory.
What caused human brain size to increase is simple: bipedalism, tools, fire, cooking, meat eating which then led to big brains. The first sign of big brains were noticed right around the time erectus had control of fire. This is no coincidence.
Bipedalism, cooking, and food drove the evolution of the human brain. Climate only has an effect on it insofar as certain foods will be available at certain latitudes. These three events in human history were the most important for the evolution of our brains. When thinking about what was happening physiologically and nutritionally around that time, the rebuttal to the statement of “Intelligence requires tools” is tools require bipedalism and further tools require bigger brains as human brains may have evolved to increase expertise capacity and not IQ (more on that in the future), which coincides with the three events outlined here. Whatever the case may be, the evolution of human intelligence is extremely interesting and is most definitely multifaceted.
Photo 10 mysteries about prehistoric humans
But what causes our brains to get bigger and bigger? One possibility is that brain development helps our ancestors to create good tools. Another reason is that, with a large brain, it can help people communicate more easily. It is also possible, the constantly changing circumstances also force our ancestors to constantly change the world and evolve the bigger brain.
9. Why do people use their legs to go?
Before our ancestors knew how to use stone tools or evolve into larger brains, humans stood up straight with their legs . But the problem is: Why do humans evolve in the direction of standing with two legs, but our other relatives have to use four limbs?
Photo 2 10 mysteries about prehistoric humans
Scientists think that, as a two-legged animal, it is possible that exercise will reduce energy more than four limbs many times. Liberation or hands have allowed our ancestors to be convenient when carrying food. Standing upright, can even help people more convenient in keeping heat, reducing the area of skin contact with the sun.
8. Where did human hair disappear?
Compared to our other hairy relatives, the appearance of the human body can be considered unique. Why do we develop in this direction? One explanation is that when our ancestors crossed the hot African steppe to explore, the removal of the outer coat was the best heat dissipation.
Another explanation is that reducing the coat is a way to reduce parasites and other infectious diseases. There is an even more extreme view that when our ancestors lived in the water, it developed into a hairless body, but compared to aquatic mammals, they still had a layer. Thick fur to cover the body.
7. Why are our relatives most extinct?
About 24,000 years ago, Homo sapiens 'smart people' were not the only species that existed in the world. The ' relatives ' closest to us, the Neanderthals are still not completely extinct. In Indonesia, it was discovered that ' Hobbit' , a branch of humans, could live up to 12,000 years ago.
Photo 3 10 mysteries about prehistoric humans
Why are they extinct but our branch still exists? Is it because of certain infectious diseases or extreme circumstances that have caused them to become extinct? Or because the 'advanced and noble' people we have destroyed those underdeveloped people?
6. Are people still evolving?
Recent findings show that people are not only evolving but also evolving very quickly. Since agricultural technology has been popularized, human evolution has been hundreds of times more than the average level of history. There are some scientists who disagree with this view.
They argue that these figures are still not sufficient to determine whether genes give us the ability to adapt to the situation. If human evolution is growing rapidly, should we ask why? Eating and illness will be pressures that force people to change their behaviors and practices.
5. What is a hobbit?
The Hobbit - a small breed discovered in 2003 on the Indonesian island of Flores. Can a species that is really extinct be called the Flores population? Are these bones of deformed Homo sapiens ?
Are they another branch of humanity that has become extinct? They may be similar to chimpanzees, but close but far apart. Solving this answer can help us understand more about human basic evolution.
Photo 4 10 mysteries about prehistoric humans
4. Why modern people originate in Africa
About 50,000 years ago, people began to expand to the outside world, beyond Africa and spread all over the world (except Antarctica), including the remote islands of the Pacific Ocean. There is a surmised scientist, this migration may be related to gene mutations.
This mutation has changed our brains, leading to people wanting to change and modernize, thus giving us the ability to use complex language and tools, build society and technology. human art. Another more common view is that the behavior of change and modernization appeared very early, before people left Africa, when the population in Africa increased too high, people were forced to leave to find new lands - and the human revolution has begun.
3. Do we have junk with Neanderthals?
Can we just mate in the line? Is there a DNA of relatives in our bodies? There are speculative scientists, maybe the Neanderthals are not really extinct but are assimilated by our humans.
2. Who was the first primitive?
Scientists have discovered that many bipedal animals can be called primitive people , including the immediate ancestors of humans or relatives. Scientists are trying to find out what is the earliest primitive, to answer many of the concerns about human evolution: How adaptation has taken place for people to become People today, what steps have taken place?
Photo 5 10 mysteries about prehistoric humans
1. Where does a modern person come from?
The most controversial issue with today's science is the issue of where modern humans have come from. The 'from Africa' hypothesis assumes that the evolutionary man began to come from Africa and then spread to the rest of the world, destroying fellow humans and replacing them (like a few hundred years) recently Europeans assimilated and destroyed American Indians.
The 'pluralistic' hypothesis assumes that people in different regions come from many places and gradually grow up. In different places, approaching the neighborhoods to mate and inherit each other's genetic traits , leads to the production of humanity. Now the view from 'Africa' is still the key point of view, but those who hold the 'Pluralism' perspective still hold their views on the emergence of modern people.
Height and weight evolved at different speeds in the bodies of our ancestors
A wide-ranging new study of fossils spanning over four million years suggests that stature and body mass advanced at different speeds during the evolution of hominins -- the ancestral lineage of which Homo sapiens alone still exist.
Published today in the journal Royal Society Open Science, the research also shows that, rather than steadily increasing in size, hominin bodies evolved in "pulse and stasis" fluctuations, with some lineages even shrinking.
The findings are from the largest study of hominin body sizes, involving 311 specimens dating from earliest upright species of 4.4m years ago right through to the modern humans that followed the last ice age.
While researchers describe the physical evolution of assorted hominin species as a "long and winding road with many branches and dead ends," they say that broad patterns in the data suggest bursts of growth at key stages, followed by plateaus where little changed for many millennia.
The scientists were surprised to find a "decoupling" of bulk and stature around one and a half million years ago, when hominins grew roughly 10cm taller but would not consistently gain any heft for a further million years, with an average increase of 10-15kgs occurring around 500,000 years ago.
Before this event, height and weight in hominin species appeared to evolve roughly "in concert," say the authors of this first study to jointly analyse both aspects of body size over millions of years.
"An increase solely in stature would have created a leaner physique, with long legs and narrow hips and shoulders. This may have been an adaptation to new environments and endurance hunting, as early Homo species left the forests and moved on to more arid African savannahs," says lead author Dr Manuel Will from Cambridge's Department of Archaeology, and a Research Fellow at Gonville and Caius College.
"The higher surface-to-volume ratio of a tall, slender body would be an advantage when stalking animals for hours in the dry heat, as a larger skin area increases the capacity for the evaporation of sweat."
"The later addition of body mass coincides with ever-increasing migrations into higher latitudes, where a bulkier body would be better suited for thermoregulation in colder Eurasian climates," he says.
However, Dr Will points out that, while these are valid theories, vast gaps in the fossil record continue to mask absolute truths. In fact, Will and colleagues often had to estimate body sizes from highly fragmented remains -- in some cases from just a single toe bone.
The study found body size to be highly variable during earlier hominin history, with a range of differently shaped species: from broad, gorilla-like Paranthropus to the more wiry or 'gracile' Australopithecus afarensis. Hominins from four million years ago weighed a rough average of 25kg and stood at 125-130cm.
As physicality morphs over deep time, increasingly converging on larger body sizes, the scientists observe three key "pulses" of significant change.
The first occurs with the dawn of our own defined species bracket, Homo, around 2.2-1.9m years ago. This period sees a joint surge in both height (around 20 cm) and weight (between 15-20kg).
Stature then separated from heft with a height increase alone of 10cm between 1.4-1.6m years ago, shortly after the emergence of Homo erectus. "From a modern perspective this is where we see a familiar stature reached and maintained. Body mass, however, is still some way off," explains Will.
It's not until a million years later (0.5-0.4m years ago) that consistently heavier hominins appear in the fossil record, with an estimated 10-15kg greater body mass signalling adaptation to environments north of the Mediterranean.
"From then onwards, average body height and weight stays more or less the same in the hominin lineage, leading ultimately to ourselves," says Will.
There are, however, a couple of exceptions to this grand narrative: Homo naledi and Homo floresiensis*. Recently discovered remains suggest these species swam against the tide of increasing body size through time.
"They may have derived from much older small-bodied ancestors, or adapted to evolutionary pressures occurring in small and isolated populations," says Will. Floresiensis was discovered on an Indonesian island.
"Our study shows that, other than these two species, hominins that appear after 1.4m years ago are all larger than 140cm and 40kg. This doesn't change until human bodies diversify again in just the last few thousand years."
"These findings suggest extremely strong selective pressures against small body sizes which shifted the evolutionary spectrum towards the larger bodies we have today."
Will and colleagues say evolutionary pressures that may have contributed include 'cladogenesis': the splitting of a lineage, with one line -- the smaller-bodied one, in this case -- becoming extinct, perhaps as a result of inter-species competition.
They also suggest that sexual dimorphism -- the physical distinction between genders, with females typically smaller in mammals -- was more prevalent in early hominin species but then steadily ironed out by evolution.
Study co-author Dr Jay Stock, also from Cambridge's Department of Archaeology, suggests this growth trajectory may continue.
"Many human groups have continued to get taller over just the past century. With improved nutrition and healthcare, average statures will likely continue to rise in the near future. However, there is certainly a ceiling set by our genes, which define our maximum potential for growth," Stock says.
"Body size is one of the most important determinants of the biology of every organism on the planet," adds Will. "Reconstructing the evolutionary history of body size has the potential to provide us with insights into the development of locomotion, brain complexity, feeding strategies, even social life."
*Both Homo naledi and Homo floresiensis are of a surprisingly young age, says Will: between