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The diversity of mammals on Earth exploded straight after the dinosaur extinction event, according to UCL researchers. New analysis of the fossil record shows that placental mammals, the group that today includes nearly 5000 species including humans, became more varied in anatomy during the Paleocene epoch - the 10 million years immediately following the event.

Senior author, Dr Anjali Goswami (UCL Genetics, Evolution & Environment), said: "When dinosaurs went extinct, a lot of competitors and predators of mammals disappeared, meaning that a great deal of the pressure limiting what mammals could do ecologically was removed. They clearly took advantage of that opportunity, as we can see by their rapid increases in body size and ecological diversity. Mammals evolved a greater variety of forms in the first few million years after the dinosaurs went extinct than in the previous 160 million years of mammal evolution under the rule of dinosaurs."

The Natural Environment Research Council-funded research, published today in the Biological Journal of the Linnean Society, studied the early evolution of placental mammals, the group including elephants, sloths, cats, dolphins and humans. The scientists gained a deeper understanding of how the diversity of the mammals that roamed the Earth before and after the dinosaur extinction changed as a result of that event.

Placental mammal fossils from this period have been previously overlooked as they were hard to place in the mammal tree of life because they lack many features that help to classify the living groups of placental mammals. Through recent work by the same team at UCL, this issue was resolved by creating a new tree of life for placental mammals, including these early forms, which was described in a study published in Biological Reviews yesterday.

First author of both papers, Dr Thomas Halliday (UCL Earth Sciences and Genetics, Evolution & Environment), said: "The mass extinction that wiped out the dinosaurs 66 million years ago is traditionally acknowledged as the start of the 'Age of Mammals' because several types of mammal appear for the first time immediately afterwards.

"Many recent studies suggest that little changed in mammal evolution during the Paleocene but these analyses don't include fossils from that time. When we look at the mammals that were present, we find a burst of evolution into new forms, followed by specialisation that finally resulted in the groups of mammals we see today. The earliest placental mammal fossils appear only a few hundred thousand years after the mass extinction, suggesting the event played a key role in diversification of the mammal group to which we belong."

The team studied the bones and teeth of 904 placental fossils to measure the anatomical differences between species. This information was used to build an updated tree of life containing 177 species within Eutheria (the group of mammals including all species more closely related to us than to kangaroos) including 94 from the Paleocene - making it the tree with the largest representation from Paleocene mammals to date. The new tree was analysed in time sections from 140 million years ago to present day, revealing the change in the variety of species.

Three different methods were used by the team to investigate the range and variation of the mammals present and all showed an explosion in mammal diversity after the dinosaur extinction. This is consistent with theories that mammals flourished when dinosaurs were no longer hunting them or competing with them for resources.

Dr Anjali Goswami (UCL Genetics, Evolution & Environment), added: "Extinctions are obviously terrible for the groups that go extinct, non-avian dinosaurs in this case, but they can create great opportunities for the species that survive, such as placental mammals, and the descendants of dinosaurs: birds."

Professor Paul Upchurch (UCL Earth Sciences), co-author of the Biological Reviews study, added: "Several previous methodological studies have shown that it is important to include as many species in an evolutionary tree as possible: this generally improves the accuracy of the tree. By producing such a large data set, we hope that our evolutionary tree for Paleocene mammals is more robust and reliable than any of the previous ones. Moreover, such large trees are very useful for future studies of large-scale evolutionary patterns, such as how early placental mammals dispersed across the continents via land bridges that no longer exist today."

The team are now investigating rates of evolution in these mammals, as well as looking at body size more specifically. Further work will involve building data from DNA into these analyses, to extend these studies to modern mammals.

Source: University College London [December 21, 2015]

Our ancestors evolved three times faster in the 10 million years after the extinction of the dinosaurs than in the previous 80 million years, according to UCL researchers.

The team found the speed of evolution of placental mammals -- a group that today includes nearly 5000 species including humans -- was constant before the extinction event but exploded after, resulting in the varied groups of mammals we see today.

Lead researcher, Dr Thomas Halliday (UCL Genetics, Evolution & Environment), said: "Our ancestors -- the early placental mammals - benefitted from the extinction of non-avian dinosaurs and dwindling numbers of competing groups of mammals. Once the pressure was off, placental mammals suddenly evolved rapidly into new forms.

"In particular, we found a group called Laurasiatheria quickly increased their body size and ecological diversity, setting them on a path that would result in a modern group containing mammals as diverse as bats, cats, rhinos, whales, cows, pangolins, shrews and hedgehogs."

The team found that the last common ancestor for all placental mammals lived in the late Cretaceous period, about three million years before the non-avian dinosaurs became extinct 66 million years ago. This date is 20 million years younger than suggestions from previous studies which used molecular data from living mammals and assumed a near-constant rate of evolution.

In this study, funded by the Natural Environment Research Council and published in >Proceedings B of the Royal Society, the researchers analysed fossils from the Cretaceous to the present day, and used the dates of their occurrence in the fossil record to estimate the timing of divergences based on an updated tree of life. The new tree was released by the same team in 2015 and has the largest representation of Paleocene mammals to date.

The scientists measured all the small changes in the bones and teeth of 904 placental fossils and mapped the anatomical differences between species on the tree of life. From measuring the number of character changes over time for each branch, they found the average rate of evolution for early placental mammals both before and after the dinosaur extinction event. They compared the average rate of evolution over the geological stages before the extinction and the geological stages after to see what impact it had.

Senior author, Professor Anjali Goswami (UCL Genetics, Evolution & Environment and UCL Earth Sciences), said: "Our findings refute those of other studies which overlooked the fossils of placental mammals present around the last mass extinction. Using rigorous methods, we've successfully tracked the evolution of early placental mammals and reconstructed how it changed over time. While the rate differed between species, we see a clear and massive spike in the rates of evolution straight after the dinosaurs become extinct, suggesting our ancestors greatly benefitted from the demise of the dinosaurs. The huge impact of the dinosaur extinction on the evolution of our ancestors really shows how important this event was in shaping the modern world."

Professor Paul Upchurch (UCL Earth Sciences), co-author of the study, added: "Our large and refined data set allows us to build a clearer picture of evolutionary history. We plan on using it to study other large-scale evolutionary patterns such as how early placental mammals dispersed across the continents via land bridges that no longer exist today."

Source: University College London [June 28, 2016]

Mammals immortalise their genes through eggs and sperm to ensure future generations inherit good quality mitochondria to power the body's cells, according to new UCL research.

Before now, it was not known why mammals rely on dedicated sex cells that are formed early in development (a germline) to make offspring whereas plants and other simple animals, such as corals and sponges, use sex cells produced later in life from normal body tissues.

In a new study, published today in >PLOS Biology and funded by Natural Environment Research Council, Engineering & Physical Sciences Research Council and the Leverhulme Trust, UCL scientists developed an evolutionary model to investigate how these differences evolved over time and discovered that the germline in mammals developed in response to selection on mitochondria (the powerhouses of cells).

First author and UCL PhD student, Arunas Radzvilavicius, said: "There have been many theories about why mammals have a specialised germline when plants and other ancient animals don't. Some suggest it was due to complexity of tissues or a selfish conflict between cells. The distinction between sex cells and normal body tissues seems to be necessary for the evolution of very complex specialised tissues like brain.

"Surprisingly, we found that these aren't the reason. Rather, it's about the number of genetic mutations in mitochondrial DNA over time, which differs between organisms, and the variation between cells caused by the mitochondria being randomly partitioned into daughter cells at each division."

In plants, mitochondrial mutations creep in slowly, so a germline isn't needed as mutations are corrected by natural selection. Mitochondrial variation is maximised by forming the next generation from the same cells used to make normal tissue cells. When the cells divide to form new daughter cells, some receive more mutant mitochondria than others and these cells are then removed through natural selection, preserving the reproductive cells containing higher quality mitochondria.

In mammals, genetic errors in mitochondria accumulate more quickly due to our higher metabolic rate so using cells that have undergone lots of division cycles would be a liability. Mitochondria are therefore only passed along to the next generation through a dedicated female germline in the form of large eggs. This protects against errors being introduced as eggs undergo many fewer replication cycles than cells in other tissues such as the gut, skin and blood.

The germline ensures that the best quality mitochondria are transferred but restricts the genetic variation in the next generation of cells in the developing embryo. This is corrected for by mammals generating far too many egg cells which are removed during development. For example, humans are born with over 6 million egg-precursor cells, 90% of which are culled by the start of puberty in a mysterious process called atresia.

Senior author, Dr Nick Lane (UCL CoMPLEX and Genetics, Evolution & Environment) added: "We think the rise in mitochondrial mutation rate likely occurred in the Cambrian explosion 550 million years ago when oxygen levels rose. This was the first appearance of motile animals in the fossil record, things like trilobites that had eyes and armour plating - predators and prey. By moving around they used their mitochondria more and that increased the mutation rate. So to avoid these mutations accumulating they needed to have fewer rounds of cell division, and that meant sequestering a specialized germline."

Co-author, Professor Andrew Pomiankowski (UCL Genetics, Evolution & Environment), concluded: "Without a germline, animals with complex development and brains could not exist. Scientists have long tried to explain the evolution of the germline in terms of complexity. Who would have thought it arose from selection on mitochondrial genes? We hope our discovery will transform the way researchers understand animal development, reproduction and aging."

Source: University College London [December 20, 2016]

Marine turtles experienced an evolutionary windfall thanks to a mass extinction of crocodyliforms around 145 million years ago, say researchers.

Crocodyliforms comprise modern crocodiles and alligators and their ancient ancestors, which were major predators that thrived on Earth millions of years ago. They evolved into a variety of species including smaller ones that lived on land through to mega-sized sea-swimming species that were up to 12 metres long. However, around 145 million years ago crocodyliforms, along with many other species, experienced a severe decline - an extinction event during a period between two epochs known as the Jurassic/Cretaceous boundary.

Now a PhD student and his colleagues from Imperial College London and University College London have carried out an extensive analysis of 200 species of crocodyliforms from a fossil database. One of the findings of the study is that the timing of the extinction coincided with the origin of modern marine turtles. The team suggest that the ecological pressure may have been lifted from early marine turtle ancestors due to the extinction of many marine crocodyliforms, which were one of their primary predators.

Jon Tennant, lead author of the study from the Department of Earth Science and Engineering at Imperial, said: "This major extinction of crocodyliforms was literally a case of out with the old and in with the new for many species. Marine turtles, the gentle, graceful creatures of the sea, may have been one of the major winners from this changing of the old guard. They began to thrive in oceans around the world when their ferocious arch-predators went into terminal decline."

In the study, published in the journal Proceedings of the Royal Society B, the researchers point to evidence in the records of a dramatic extinction of crocodyliforms during the Jurassic/Cretaceous boundary. Up to 80 per cent of species on land and in marine environments were wiped out. This decline was primarily due to a drop in sea levels, which led to a closing off of shallow marine environments such as lagoons and coastal swamps. These were the homes and primary hunting grounds for many crocodyliforms.

The decimation of many marine crocodyliforms may also have laid the way for their ecological replacement by other large predatory groups such as modern shark species and new types of plesiosaurs. Plesiosaurs were long-necked, fat-bodied and small-headed ocean-going creatures with fins, which later went extinct around 66 million years ago.

Other factors that contributed to the decline of marine crocodyliforms included a change in the chemistry of ocean water with increased sulphur toxicity and a depletion of oxygen.

While primitive crocodyliform species on land also suffered major declines, the remaining species diversified into new groups such as the now extinct notosuchians, which were much smaller in size at around 1.5 metres in length. Eusuchians also came to prominence after the extinction, which led to today's crocodiles.

To carry out the study on crocodyliforms the team used the Paleobiology Database, which is a professionally curated digital archive of all known fossil records. The team analysed almost 1,200 crocodyliform fossil records.

Scientists have known since the early 1970s about the Jurassic/Cretaceous boundary extinction from fossil records. However, researchers have focussed on other extinction events and as a consequence less has been done to understand in detail the effects of Jurassic/Cretaceous boundary extinction on species like crocodyliforms.

The next steps will see the analysis extended to other groups including dinosaurs, amphibians and mammals to learn more about the effects of the Jurassic/Cretaceous boundary on their biodiversity

Source: Imperial College London [March 09, 2016]

Human teeth discovered in southern China provide evidence that our species left the African continent up to 70,000 years earlier than prevailing theories suggest, a study published on Wednesday said.

Homo sapiens reached present-day China 80,000-120,000 years ago, according to the study, which could redraw the migration map for modern humans.

"The model that is generally accepted is that modern humans left Africa only 50,000 years ago," said Maria Martinon-Torres, a researcher at University College London and a co-author of the study.

"In this case, we are saying the H. sapiens is out of Africa much earlier," she told the peer-reviewed journal Nature, which published the study.

While the route they travelled remains unknown, previous research suggests the most likely path out of East Africa to east Asia was across the Arabian Peninsula and the Middle East.

The findings also mean that the first truly modern humans -- thought to have emerged in east Africa some 200,000 years ago -- landed in China well before they went to Europe.

There is no evidence to suggest that H. sapiens entered the European continent earlier than 45,000 years ago, at least 40,000 years after they showed up in present-day China.

The 47 teeth exhumed from a knee-deep layer of grey, sandy clay inside the Fuyan Cave near the town of Daoxian closely resemble the dental gear of "contemporary humans," according to the study.

They could only have come from a population that migrated from Africa, rather than one that evolved from an another species of early man such as the extinct Homo erectus, the authors said.

The scientists also unearthed the remains of some 38 mammals, including specimens of five extinct species, one of them a giant panda larger than those in existence today.

No tools were found.

"Judging by the cave environment, it may not have been a living place for humans," lead author Wu Liu from the Chinese Academy of Science in Beijing told AFP.

The study, published in the journal Nature, also rewrites the timeline of early man in China.

Up to now, the earliest proof of H. sapiens east of the Arabian Peninsula came from the Tianyuan Cave near Beijing, and dated from no more than 40,000 years ago.

The new discovery raises questions about why it took so long for H. sapiens to find their way to nearby Europe.

"Why is it that modern humans -- who were already at the gates -- didn't really get into Europe?", Martinon-Torres asked.

Wu and colleagues propose two explanations.

The first is the intimidating presence of Neanderthal man. While this species of early human eventually died out, they were spread across the European continent up until at least some 50,000 years ago.

"The classic idea is that H. sapiens... took over the Neanderthal empire, but maybe Neanderthals were a kind of ecological barrier, and Europe was too small a place" for both, Martinon-Torres said.

Another impediment might have been the cold.

Up until the Ice Age ended 12,000 years ago, ice sheets stretched across a good part of the European continent, a forbidding environment for a new species emerging from the relative warmth of East Africa.

"H. sapiens originated in or near the tropics, so it makes sense that the species' initial dispersal was eastwards rather than northwards, where winter temperatures rapidly fell below freezing," Robin Dennell of the University of Exeter said in a commentary, also in Nature.

Martinon-Torres laid out some of the questions to be addressed in future research, using both genetics and fossil records.

"What are the origins of these populations, and what was their fate? Did they vanish? Could they be the ancestors of later and current populations that entered Europe?"

She also suggested there might have been "different movements and migrations" out of Africa, not just one.

Besides the prehistoric panda, called Ailuropoda baconi, the scientists found an extinct species of a giant spotted hyaena.

An elephant-like creature called Stegodon orientalis and a giant tapir, also present, were species that may have survived into the era when the Chinese had developed writing, some 3500 years ago.

The cache of teeth nearly went unnoticed, Wu told AFP.

He and his Chinese colleagues discovered the cave -- and its menagerie of long-deceased animals -- in the 1980s, but had no inkling that it also contained human remains.

But 25 years later, while revisiting the site, Wu had a hunch.

"By thinking about the cave environment, we realised that human fossils might be found there," he told AFP by email. "So we started a five-year excavation."

Author: Marlowe Hood | Source: AFP [October 14, 2015]