̽��ֱ�� of Cambridge - Manuel Will

Height and weight evolved at different speeds in the bodies of our ancestors

fpjl2 — Wed, 08 Nov 2017 00:33:01 +0000

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.

̽��ֱ��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.

̽��ֱ��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 & Caius College.

“ ̽��ֱ��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.

“ ̽��ֱ��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.

̽��ֱ��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.

̽��ֱ��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.”

̽��ֱ��largest study to date of body sizes over millions of years finds a “pulse and stasis” pattern to hominin evolution, with surges of growth in stature and bulk occurring at different times. At one stage, our ancestors got taller around a million years before body mass caught up.

Body size is one of the most important determinants of the biology of every organism on the planet

Manuel Will

̽��ֱ�� of Cambridge

Femoral head bones of different hominin species. From top to bottom: Australopithecus afarensis (4-3 million years; ~40 kg, 130 cm); Homo ergaster (1.9-1.4 million years; 55-60 kg; ~165 cm); Neanderthal (200.000-30.000 years; ~70 kg; ~163 cm).

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Earliest humans had diverse range of body types, just as we do today

jeh98 — Fri, 27 Mar 2015 09:14:54 +0000

One of the dominant theories of our evolution is that our genus, Homo, evolved from small-bodied early humans to become the taller, heavier and longer legged Homo erectus that was able to migrate beyond Africa and colonise Eurasia. While we know that small-bodied Homo erectus – averaging less than five foot and under eight stone – were living in Georgia in southern Europe by 1.77 million years ago, the timing and geographic origin of the larger body size that we associate with modern humans has, until now, remained unresolved.

But a joint study by researchers at the Universities of Cambridge and Tübingen (Germany), published today in the Journal of Human Evolution, has now shown that the main increase in body size occurred tens of thousands of years after Homo erectus left Africa, and primarily in the Koobi Fora region of Kenya. According to Manuel Will, a co-author of the study from the Department of Early Prehistory and Quaternary Ecology at Tübingen, “the evolution of larger bodies and longer legs can thus no longer be assumed to be the main driving factor behind the earliest excursions of our genus to Eurasia”.

Researchers say the results from a new research method, using tiny fragments of fossil to estimate our earliest ancestors’ height and body mass, also point to the huge diversity in body size we see in humans today emerging much earlier than previously thought.

“What we’re seeing is perhaps the beginning of a unique characteristic of our own species – the origins of diversity,” said Dr Jay Stock, co-author of the study from the ̽��ֱ�� of Cambridge’s Department of Archaeology and Anthropology. “It’s possible to interpret our findings as showing that there were either multiple species of early human, such as Homo habilis, Homo ergaster and Homo rudolfensis, or one highly diverse species. This fits well with recent cranial evidence for tremendous diversity among early members of the genus Homo.”

“If someone asked you ‘are modern humans 6 foot tall and 70kg?’ you’d say ‘well some are, but many people aren’t,’ and what we’re starting to show is that this diversification happened really early in human evolution,” said Stock.

̽��ֱ��study is the first in 20 years to compare the body size of the humans who shared the earth with mammoths and sabre-toothed cats between 2.5 and 1.5 million years ago. It is also the first time that many fragmentary fossils – some as small as toes and tiny ankle bones no more than 5cm long – have been used to make body size estimates.

Comparing measurements of fossils from sites in Kenya, Tanzania, South Africa, and Georgia, the researchers found that there was significant regional variation in the size of early humans during the Pleistocene. Some groups, such as those who lived in South African caves, averaged 4.8 feet tall; some of those found in Kenya’s Koobi Fora region would have stood at almost 6 foot, comparable to the average of today´s male population in Britain.

“Basically every textbook on human evolution gives the perspective that one lineage of humans evolved larger bodies before spreading beyond Africa. But the evidence for this story about our origins and the dispersal out of Africa just no longer really fits,” said Stock. “ ̽��ֱ��first clues came from the site of Dmanisi in Georgia where fossils of really small-bodied people date to 1.77 million years ago. This has been known for several years, but we now know that consistently larger body size evolved in Eastern Africa after 1.7 million years ago, in the Koobi Fora region of Kenya.”

“We tend to simplify our interpretations because the fossil record is patchy and we have to explain it in some way. But revealing the diversity that exists is just as important as those broad, sweeping explanations.”

Previous studies have been based on small samples of only 10-15 fossils because techniques for calculating the height and body mass of individuals required specific pieces of bone such as the hip joint or most of a leg bone. Stock and Will have used a sample size three times larger, estimating body size for over 40 specimens contained in collections all over Africa and Georgia, making it the largest comparative study conducted so far.

Instead of waiting for new fossils to be discovered and hoping that they contained these specific bones, Stock and Will decided to try a different approach and make use of previously over-looked fossils.

In what Stock describes as a “very challenging project,” they spent a year developing new equations that allowed them to calculate the height and body mass of individuals using much smaller bones, some as small as toes. By comparing these bones to measurements taken from over 800 modern hunter-gatherer skeletons from around the world and applying various regression equations, the researchers were able to estimate body size for many new fossils that have never been studied in this way before.

“In human evolution we see body size as one of the most important characteristics, and from examining these ‘scrappier’ fossils we can get a much better sense of when and where human body size diversity arose. Before 1.7 million years ago our ancestors were seldom over 5 foot tall or particularly heavy in body mass.

“When this significant size shift to much heavier, taller individuals happened, it occurred primarily in one particular place – in a region called Koobi Fora in northern Kenya around 1.7 million years ago. That means we can now start thinking about what regional conditions drove the emergence of this diversity, rather than seeing body size as a fixed and fundamental characteristic of a species,” said Stock.

Inset images – the landscape of the West Turkana region of Kenya where the 'Nariokotome boy' skeleton was discovered, credit Manuel Will; table of estimated heights and weights of early Homo during the Pleistocene.

New research harnessing fragmentary fossils suggests our genus has come in different shapes and sizes since its origins over two million years ago, and adds weight to the idea that humans began to colonise Eurasia while still small and lightweight.

What we’re seeing is perhaps the beginning of a unique characteristic of our own species – the origins of diversity.

Jay Stock

Cast of the 'Nariokotome boy' (Homo ergaster) skeleton

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