̽��ֱ�� of Cambridge - Pleistocene

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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1.5 million years of climate history revealed after scientists solve mystery of the deep

bjb42 — Fri, 10 Aug 2012 02:51:00 +0000

Scientists have announced a major breakthrough in understanding the Earth’s climate machine by reconstructing highly accurate records of changes in ice volume and deep-ocean temperatures over the last 1.5 million years.

̽��ֱ��study, which is reported in the journal Science, offers new insights into a decades-long debate about how the shifts in the Earth’s orbit relative to the sun have taken the Earth into and out of an ice-age climate.

Being able to reconstruct ancient climate change is a critical part of understanding why the climate behaves the way it does. It also helps us to predict how the planet might respond to man-made changes, such as the injection of large quantities of carbon dioxide into the atmosphere, in the future.

Unfortunately, scientists trying to construct an accurate picture of how such changes caused past climatic shifts have been thwarted by the fact that the most readily available marine geological record of ice-ages – changes in the ratio of oxygen isotopes (Oxygen 18 to Oxygen 16) preserved in tiny calcareous deep sea fossils called foraminifera – is compromised.

This is because the isotope record shows the combined effects of both deep sea temperature changes, and changes in the amount of ice volume. Separating these has in the past proven difficult or impossible, so researchers have been unable to tell whether changes in the Earth’s orbit were affecting the temperature of the ocean more than the amount of ice at the Poles, or vice-versa.

̽��ֱ��new study, which was carried out by researchers in the ̽��ֱ�� of Cambridge Department of Earth Sciences, appears to have resolved this problem by introducing a new set of temperature-sensitive data. This allowed them to identify changes in ocean temperatures alone, subtract that from the original isotopic data set, and then build what they describe as an unprecedented picture of climatic change over the last 1.5 million years – a record of changes in both oceanic temperature and global ice volume.

Included in this is a much fuller representation of what happened during the “Mid-Pleistocene Transition” (MPT) - a major change in the Earth’s climate system which took place sometime between 1.25 million and 600 thousand years ago. Before the MPT, the alternation between glacial periods of extreme cold, and warmer interglacials, happened at intervals of approximately 41,000 years. After the MPT, the major cycles became much longer, regularly taking 100,000 years. ̽��ֱ��second pattern of climate cycles is the one we are in now. Interestingly, this change occurred with little or no orbital forcing.

“Previously, we didn’t really know what happened during this transition, or on either side of it,” Professor Harry Elderfield, who led the research team, said. “Before you separate the ice volume and temperature signals, you don’t know whether you’re seeing a climate record in which ice volume changed dramatically, the oceans warmed or cooled substantially, or both.”

“Now, for the first time, we have been able to separate these two components, which means that we stand a much better chance of understanding the mechanisms involved. One of the reasons why that is important, is because we are making changes to the factors that influence the climate now. ̽��ֱ��only way we can work out what the likely effects of that will be in detail is by finding analogues in the geological past, but that depends on having an accurate picture of the past behaviour of the climate system.”

Researchers have developed more than 30 different models for how these features of the climate might have changed in the past, in the course of a debate which has endured for more than 60 years since pioneering work by Nobel Laureate Harold Urey in 1946. ̽��ֱ��new study helps resolve these problems by introducing a new dataset to the picture - the ratio of magnesium (Mg) to calcium (Ca) in foraminifera. Because it is easier for magnesium to be incorporated at higher temperatures, larger quantities of magnesium in the tiny marine fossils imply that the deep sea temperature was higher at that point in geological time.

̽��ֱ��Mg/Ca dataset was taken from the fossil record contained in cores drilled on the Chatham Rise, an area of ocean east of New Zealand. It allowed the Cambridge team to map ocean temperature change over time. Once this had been done, they were able to subtract that information from the oxygen isotopic record. “ ̽��ֱ��calculation tells us the difference between what water temperature was doing and what the ice sheets were doing across a 1.5 million year period,” Professor Elderfield explained.

̽��ֱ��resulting picture shows that ice volume has changed much more dramatically than ocean temperatures in response to changes in orbital geometry. Glacial periods during the 100,000-year cycles have been characterised by a very slow build-up of ice which took thousands of years, the result of ice volume responding to orbital change far more slowly than the ocean temperatures reacted. Ocean temperature change, however, reached a lower limit, probably because the freezing point of sea water put a restriction on how cold the deep ocean could get.

In addition, the record shows that the transition from 41,000-year cycles to 100,000-year cycles, the characteristic changeover of the MPT, was not as gradual as previously thought. In fact, the build-up of larger ice sheets, associated with longer glacials, appears to have begun quite suddenly, around 900,000 years ago. ̽��ֱ��pattern of the Earth’s response to orbital forcing changed dramatically during this “900,000 year event”, as the paper puts it.

̽��ֱ��research team now plan to apply their method to the study of deep-sea temperatures elsewhere to investigate how orbital changes affected the climate in different parts of the world.

“Any uncertainty about the Earth’s climate system fuels the sense that we don’t really know how the climate is behaving, either in response to natural effects or those which are man-made,” Professor Elderfield added. “If we can understand how earlier changes were initiated and what the impacts were, we stand a much better chance of being able to predict and prepare for changes in the future.”

Study successfully reconstructed temperature from the deep sea to reveal how global ice volume has varied over the glacial-interglacial cycles of the past 1.5 million years.

̽��ֱ��only way we can work out what the likely effects of the changes we are making to the climate will be is by finding analogues in the geological past. That depends on having an accurate picture of the past behaviour of the climate system.

Harry Elderfield

Julian Dowdeswell.

Tabular iceberg. ̽��ֱ��production of tabular icebergs is a major mechanism of mass loss from the Antarctic Ice Sheet.

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