Evolution

Fearless fowl grow and lay better

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Animal populations that humans selected to domesticate grew increasingly tame

Date:
September 16, 2015
Source:
Linköping University
Summary:
A reduced fear of humans can be the driving force behind the characteristics that have developed since wild animals became domesticated, according to research by ethologists.

About 8,000 years ago we began to domesticate animals — a process that fundamentally changed the way animals and people live. Domesticated animals of today have characteristics that distinguish them from their wild ancestors, including size, colour, reproduction and behaviour.

In a fresh study the LiU researchers show that many of these changes can have been driven by a simple fact: the animal populations that humans selected to domesticate grew increasingly tame. The study is now published in Biology Letters.

The researchers used a population of red junglefowl (Gallus gallus), the wild ancestor of all domesticated fowl. For five generations they selected animals with a congenital reduced fear of humans, and bred their offspring. For comparison, they also bred a separate line from the fowl that were most fearful of humans.

“We used a standardised behaviour test where we studied the fowl’s reaction to a human. This method resembled the conditions during the very first stage of fowl husbandry 8,000 years ago,” says Beatrix Agnvall, doctoral student in ethology and first author of the article.

After just five generations, the increasingly tame fowl had developed a higher metabolism and feed conversion rate — they grew more although they ate less than the more fearful animals in the control group. They were also more cautious in situations where humans were not involved, and, as in previous studies of the same animals, they laid larger eggs. The levels of the hormone serotonin were higher in the tame roosters, and the researchers believe that this can be one of the mechanisms driving the results.

According to Per Jensen, professor of ethology at LiU and head of the study, increased tameness was an important prerequisite in the animals’ ability to live with humans.

“The results show that it can automatically have led to many of the characteristics that we and our ancestors liked about domesticated animals. Therefore we can suppose that our ancestors didn’t necessarily select animals because they were good at producing food, but mainly because they were easy to manage,” says Prof Jensen, who believes the results could also apply to other domesticated animals like pigs, sheep and cattle.

Story Source:

The above post is reprinted from materials provided by Linköping University. Note: Materials may be edited for content and length.

Journal Reference:

  1. B Agnvall, R Katajamaa, J Altimiras & P Jensen. Is domestication driven by reduced fear of humans? Boldness, metabolism and serotonin levels in divergently selected red junglefowl (Gallus gallus). Biology Letters, September 2015 DOI: 10.1098/rsbl.2015.0509

Shift in human ancestors’ diet earlier than previously thought

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Key move to grass-based foods was 400,000 years earlier than previously known

Date:
September 15, 2015
Source:
Johns Hopkins University
Summary:
Pre-humans’ shift toward a grass-based diet took place about 400,000 years earlier than experts previously thought, providing a clearer picture of a time of rapid change in conditions that shaped human evolution.

Millions of years ago, our primate ancestors turned from trees and shrubs to search for food on the ground. In human evolution, that has made all the difference.

The shift toward a grass-based diet marked a significant step toward the diverse eating habits that became a key human characteristic, and would have made these early humans more mobile and adaptable to their environment.

New evidence just published by a research team led by a Johns Hopkins University scientist shows that this significant shift took place about 400,000 years earlier than experts previously thought, providing a clearer picture of a time of rapid change in conditions that shaped human evolution.

Naomi E. Levin, the lead author of the report just published in Proceedings of the National Academy of Sciences, said the diet shift is one of an array of changes that took place during the Pliocene era — 2.6 million to 5.3 million years ago — when the fossil record indicates human ancestor species were starting to spend more time on the ground walking on two feet. Understanding the timing of these events can help show how one change related to another.

“A refined sense for when the dietary changes took place among early humans, in relation to changes in our ability to be bipedal and terrestrial, will help us understand our evolutionary story,” said Levin, an assistant professor in the Department of Earth and Planetary Sciences.

The paper reports on an analysis of fossil teeth found in Ethiopia that shows the shift from a diet based on trees and shrubs to one that included grass-based foods took place about 3.8 million years ago — roughly 400,000 years earlier than the date supported by previous research. (Grass-based foods could include not only grasses and their roots, but also insects or animals that ate grass.)

The shift in eating habits would have broadened our ancestors’ horizons and improved their species’ capacity for survival, Levin said.

“You can then range wider,” Levin said of the human precursors, species including Australopithecus afarensis, extinct some 3 million years ago and represented most famously in the fossil informally known as “Lucy.” “You can be in more places, more resilient to habitat change.”

“This research reveals surprising insights into the interactions between morphology and behavior among Pliocene primates,” said co-author Yohannes Haile-Selassie of the Cleveland Museum of Natural History. “The results not only show an earlier start to grass-based food consumption among hominins and baboons but also indicate that form does not always precede function. In the earliest baboons, dietary shift toward grass occurred before its teeth were specialized for grazing.”

Researchers analyzed 152 fossil teeth from an array of animals including pigs, antelopes, giraffes and human ancestors gathered from a roughly 100 square-mile area of what is now the Afar region of Ethiopia. Among the samples were teeth from hominins — including contemporary humans and our extinct ancestors — believed to represent 16 different individuals, said Levin, one of four co-authors of the paper. Her collaborators were Haile-Selassie, Stephen R. Frost of the University of Oregon and Beverly Z. Saylor of Case Western Reserve University.

The teeth were examined for carbon isotope distribution, a marker that can distinguish the types of foods the animals ate. The data showed that both human ancestors and members of a now-extinct, large species of baboon were eating large amounts of grass-based foods as early as 3.76 million years ago. Previous research dated the earliest evidence for grass-based foods in early human diets to about 3.4 million years ago.

The researchers could not firmly establish a link between external environmental change and the diet of hominins and baboons, but instead attribute the dietary expansion to changes in relations among members of the African primate communities, such as the appearance of new species of primates.

“Timing is critical to understanding the context for this dietary expansion among early humans in relationship to what’s happening in global climate, in vegetation communities in Africa, among other mammals, and in terms of the other evolutionary changes that are happening among early humans,” she said. “If we know the timing of events we can start to relate them to one another.”

Story Source:

The above post is reprinted from materials provided by Johns Hopkins University. Note: Materials may be edited for content and length.

Journal Reference:

  1. Naomi E. Levin, Yohannes Haile-Selassie, Stephen R. Frost, Beverly Z. Saylor. Dietary change among hominins and cercopithecids in Ethiopia during the early Pliocene. Proceedings of the National Academy of Sciences, 2015; 201424982 DOI: 10.1073/pnas.1424982112

Ancient ancestor of humans with tiny brain discovered

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Date:September 10, 2015

Source:University of Colorado Anschutz Medical Campus

Summary:Scientists have discovered a new species of hominin, a small creature with a tiny brain that opens the door to a new way of thinking about our ancient ancestors.

An international team of scientists, including one from the University of Colorado Denver and another from the University of Colorado Anschutz Medical Campus in Aurora, announced the discovery Thursday of a new species of hominin, a small creature with a tiny brain that opens the door to a new way of thinking about our ancient ancestors.

The discovery of 15 individuals, consisting of 1,550 bones, represents the largest fossil hominin find on the African continent.

“We found adults and children in the cave who are members of genus Homo but very different from modern humans,” said CU Denver Associate Professor of Anthropology Charles Musiba, PhD, who took part in a press conference Thursday near the discovery inside the Rising Star Cave in the Cradle of Humankind World Heritage Site outside Johannesburg, South Africa. “They are very petite and have the brain size of chimpanzees. The only thing similar we know of are the so-called `hobbits’ of Flores Island in Indonesia.”

Homofloresiensis or Flores Man was discovered in 2003. Like this latest finding, it stood 3.5 five feet high and seems to have existed relatively recently though the exact age is unknown.

Caley Orr, PhD, an assistant professor of cell and developmental biology at the University of Colorado School of Medicine, analyzed the fossil hands.

“The hand has human-like features for manipulation of objects and curved fingers that are well adapted for climbing,” Orr said. “But its exact position on our family tree is still unknown.”

The new species has been dubbed Homo naledi after the cave where it was found — naledi means `star’ in the local South African language Sesotho.

One of the most intriguing aspects of the discovery is that the bodies appear to have been deposited in the cave intentionally. Scientists have long believed this sort of ritualized or repeated behavior was limited to humans.

The team of 35 to 40 scientists was led by Lee Berger, research professor in the Evolutionary Studies Institute at the University of Witwatersrand in South Africa. It was supported by the National Geographic Society and the National Research Foundation. The October issue of National Geographic magazine will feature the discovery as its cover story. It will also be the subject of a NOVA/National Geographic Special airing Sept. 16.

Getting inside the Dinaledi chamber of the remote cave system was difficult, requiring the help of six `underground astronauts,’ who squeezed through a 7-inch wide gap to reach the remains.

“The chamber has not given up all of its secrets,” said Berger, a National Geographic Explorer-in-Residence. “There are potentially hundreds if not thousands of remains of H. naledi still down there.”

The announcement coincides with the publication of two studies about the new species in the journal eLife, co-authored by Musiba and Orr.

In it, the researchers try to place Homonaledi in context with other species. Generally speaking, they say, there is an assumption that any new group of fossils must belong to an existing species.

But it’s not that simple here.

“Assigning these remains to any known species of Homo is problematic,” the study said. “While Homo(naledi) shares aspects of cranial and mandibular morphology with Homohabilis, Homorudolfensis, Homoerectus, MP Homo and Homosapiens, it differs from all of these taxa in its unique combination of derived cranial vault, maxillary, and mandibular morphology.”

The study suggests that Homonaledi most closely resembles Homoerectus with its small brain and body size. Yet it also resembles Australopithecus which highlights its own uniqueness.

Complicating matters is the fact that researchers still don’t know the exact age of the fossil site.

“If these fossils are late Pliocene or early Pleistocene, it is possible that this new species of small-brained, early Homorepresents an intermediate between Australopithecus and Homoerectus,” the study said.

That would also make the new species very old.

But if the fossils are more recent, they theorize, it raises the possibility that a small-brained Homolived in southern Africa at the same time as larger brained Homospecies were evolving.

“This raises many questions,” Musiba said. “How many species of human were there? Were their lines that simply extended outward and then disappeared? Did they co-exist with modern humans? Did they interbreed?”

Homonaledi has a chest similar to a chimpanzee and hands and feet proportionate with modern humans, though with curved fingers.

“They would have had great climbing ability,” said Musiba. “The oldest adults were about 45 and the youngest were infants.”

He described poring over the bones late at night as akin to `hitting the jackpot.’

“You just didn’t want to go home because it was so exciting,” he said. “I felt like a kid in a candy store.” The find represents another milestone in Musiba’s efforts to advance the understanding of our earliest human relatives.

As director of CU Denver’s Tanzania Field School, he takes groups of students each year to gain hands-on experience working in and around the famed Laetoli hominin footprints site and Olduvai Gorge where some of the oldest hominin remains have been found.

Not long ago, they discovered ancient footprints of lions, rhinos and antelopes near those of the early hominins.

And last year, Musiba was appointed to an international team of advisors dedicated to building a museum complex in Tanzania to showcase a collection of 70 hominin footprints, estimated at 3.6 million years old. They are considered the earliest example of bipedalism among hominins.

Musiba said the Rising Star expedition was notable for getting so many anthropologists to work together.

“Anthropology can be a cut-throat profession with all these scientists scrambling for limited resources,” he said. “To me one of the most exciting aspects of this research was the collaborative nature of it.”

Story Source:

The above post is reprinted from materials provided by University of Colorado Anschutz Medical Campus. The original item was written by David Kelly. Note: Materials may be edited for content and length.

Journal Reference:

  1. Paul HGM Dirks, Lee R Berger, Eric M Roberts, Jan D Kramers, John Hawks, Patrick S Randolph-Quinney, Marina Elliott, Charles M Musiba, Steven E Churchill, Darryl J de Ruiter, Peter Schmid, Lucinda R Backwell, Georgy A Belyanin, Pedro Boshoff, K Lindsay Hunter, Elen M Feuerriegel, Alia Gurtov, James du G Harrison, Rick Hunter, Ashley Kruger, Hannah Morris, Tebogo V Makhubela, Becca Peixotto, Steven Tucker. Geological and taphonomic context for the new hominin speciesHomo naledifrom the Dinaledi Chamber, South Africa. eLife, 2015; 4 DOI: 10.7554/eLife.09561

Scientists home in on origin of human, chimpanzee facial differences

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Date: September 10, 2015

Source: Stanford University Medical Center

Summary: A study of species-specific regulation of gene expression in chimps and humans has identified regions important in human facial development and variation.

The face of a chimpanzee is decidedly different from that of a human, despite the fact that the apes are our nearest relative in the primate tree. Now researchers at the Stanford University School of Medicine have begun to pinpoint how those structural differences could arise in two species with nearly identical genetic backgrounds.

The key lies in how genes involved in facial development and human facial diversity are regulated — how much, when and where the genes are expressed– rather than dissimilarities among the genes themselves. In particular, the researchers found that chimps and humans express different levels of proteins known to control facial development, including some involved in jaw and nose length and skin pigmentation.

“We are trying to understand the regulatory changes in our DNA that occurred during recent evolution and make us different from the great apes,” said Joanna Wysocka, PhD, associate professor of developmental biology and of chemical and systems biology. “In particular, we are interested in craniofacial structures, which have undergone a number of adaptations in head shape, eye placement and facial structure that allow us to house larger brains, walk upright and even use our larynx for complex speech.”

The researchers coined the term “cellular anthropology” to explain how some steps of early primate development can be mimicked in a dish, and thus used to study gene-expression changes that can shed light on our recent evolutionary past.

A study describing the research will be published online Sept. 10 in Cell. Graduate student Sara Prescott is the lead author. Wysocka and senior research scientist Tomasz Swigut, PhD, share senior authorship of the study.

The role of enhancer regions

For their comparison, the researchers focused on areas of DNA known as enhancer regions in human and chimpanzee genomes. These regions contain chemical tags and proteins bound to the DNA that control when, where and how nearby genes are expressed. Prescott and her colleagues wondered whether differences in the way proteins bind to these enhancer regions during development could explain morphological differences between humans and chimpanzees.

“We wanted to look at how the activity of these enhancer regions may have changed during recent evolution,” said Wysocka. “Many recent studies have shown that changes in the DNA sequences of enhancers may mediate morphological differences among species.”

To conduct the study, however, Prescott and her colleagues had to obtain a specialized type of cell present only in very early primate development. The cells, called cranial neural crest cells, originate in humans within about five to six weeks after conception. Although they first appear along what eventually becomes the spinal cord, the neural crest cells then migrate over time to affect facial morphology and differentiate into bone, cartilage and connective tissue of the head, and face.

“These cells are unique,” said Prescott. “If we want to understand what makes human and chimp faces different, we have to look to the source — to the cell types responsible for making these early patterning decisions. If we were to look later in development or in adult tissues, we would see differences between the species but they will tell us little about how those differences were created during embryogenesis. But accessing early cell types like neural crest cells can be quite difficult, especially when studying primates.”

To obtain this elusive cell type, the researchers used induced pluripotent stem cells, or iPS cells, made from chimpanzees. IPS cells, which are made from easy-to-obtain skin or blood samples, can be coaxed to become other tissues. Although iPS cells from humans have been well-studied, they’ve only recently been made from chimpanzees in the laboratory of Fred Gage, PhD, a professor of genetics at the Salk Institute for Biological Studies and a co-author of the study.

Prescott and her colleagues coaxed human and chimpanzee iPS cells to become cranial neural crest cells by growing them in the laboratory under a specific set of conditions. They then examined enhancer regions throughout the genome, looking for those that were not just active and therefore likely to be involved in craniofacial development, but also those whose patterns or degrees of activity seemed to vary between human and chimpanzee cells.

“Of course, humans and chimps are very closely related,” said Wysocka. “Most of the regulatory elements are the same between the two species. But we did find some differences. In particular, we found about 1,000 enhancer regions that are what we termed species-biased, meaning they are more active in one species or the other. Interestingly, many of the genes with species-biased enhancers and expression have been previously shown to be important in craniofacial development or associated with normal intrahuman facial variation.”

Snout length, shape and pigmentation

In particular, the researchers found that two genes, PAX3 and PAX7, known to affect snout length and shape in laboratory mice, as well as skin pigmentation, were expressed at higher levels in chimpanzees than in humans. Humans with less than the normal amount of PAX3 have a condition called Waardenburg syndrome, which includes craniofacial, auditory and pigmentation defects. Genomewide association studies in humans have identified PAX3 as a region involved in normal facial variation.

In contrast, another gene known to be involved in determining the shape of the beaks of finches and the jaw of a fish called a cichlid was expressed at higher levels in humans than in chimpanzees. In mice, overexpression of this gene, BMP4, in cranial neural crest cells causes a marked change in face shape, including a rounding of the skull and eyes that are more near the front of the face.

“We are now following up on some of these more interesting species-biased enhancers to better understand how they impact morphological differences,” said Wysocka. “It’s becoming clear that these cellular pathways can be used in many ways to affect facial shape.”

Another Stanford-affiliated author of the study is research assistant Rajini Srinivasan.

The research was supported by the National Institutes of Health (grants R01GMO095555 and U01DE024430), the California Institute for Regenerative Medicine, the W.M. Keck Foundation and the Innovation Fund.

Story Source:

The above post is reprinted from materials provided by Stanford University Medical Center. The original item was written by Krista Conger. Note: Materials may be edited for content and length.

Journal Reference:

  1. Sara L. Prescott, Rajini Srinivasan, Maria Carolina Marchetto, Irina Grishina, Iñigo Narvaiza, Licia Selleri, Fred H. Gage, Tomek Swigut, Joanna Wysocka. Enhancer Divergence and cis-Regulatory Evolution in the Human and Chimp Neural Crest. Cell, 2015 DOI: 10.1016/j.cell.2015.08.036

Dogs May Understand Human Point of View

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Feb. 11, 2013 — Domestic dogs are much more likely to steal food when they think nobody can see them, suggesting for the first time that dogs are capable of understanding a human’s point of view.

Many dog owners think their pets are clever or that they understand humans but, until now, this has not been tested by science.

Dr Juliane Kaminski, of the University of Portsmouth’s Department of Psychology, has shown that when a human forbids a dog from taking food, dogs are four times more likely to disobey in a dark room than a lit room, suggesting they take into account what the human can or cannot see.

Dr Kaminski said: “That’s incredible because it implies dogs understand the human can’t see them, meaning they might understand the human perspective.”

This is the first study to examine if dogs differentiate between different levels of light when they are developing strategies on whether to steal food. It is published in the journal Animal Cognition. The research was funded by the Max Planck Society, Dr Kaminski’s former employer.

Dr Kaminski said: “Humans constantly attribute certain qualities and emotions to other living things. We know that our own dog is clever or sensitive, but that’s us thinking, not them.

“These results suggest humans might be right, where dogs are concerned, but we still can’t be completely sure if the results mean dogs have a truly flexible understanding of the mind and others’ minds. It has always been assumed only humans had this ability.”

The research is an incremental step in our understanding of dogs’ ability to think and to understand which could, in turn, be of use to those working with dogs and those who keep them as pets.

Dr Kaminski ran a series of experiments in varied light conditions. In each test, a dog was forbidden by a human from taking the food. When the room was dark, the dogs took more food and took it more quickly than when the room was lit.

The tests were complex and involved many variables to rule out that dogs were basing their decisions on simple associative rules, for example, that dark means food.

There is no evidence on how well dogs can see in the dark, but the results of this research show dogs can differentiate between light and dark.

Dr Kaminski said: “The results of these tests suggest that dogs are deciding it’s safer to steal the food when the room is dark because they understand something of the human’s perspective.”

Dogs’ understanding may be limited to the here and now, rather than on any higher understanding, Dr Kaminski said, and more research is needed to identify what mechanisms are controlling dogs’ behaviour.

In total, 42 female and 42 male domestic dogs aged one year or older took part in the tests. They were chosen only if they were comfortable without their owners in the room, even in complete darkness, and if they were interested in food. “Some dogs are more interested in by food than others,” Dr Kaminski said.

Previous studies have shown chimpanzees have a sophisticated understanding and seem to know when someone else can or can’t see them and can also remember what others have seen in the past. It is not known how sophisticated dogs’ understanding is in comparison. Many earlier research papers have found that, for dogs, a human’s eyes are an important signal when deciding how to behave, and that they respond more willingly to attentive humans, than inattentive ones.

Story Source:

The above story is reprinted from materials provided byUniversity of Portsmouth.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

  1. Juliane Kaminski, Andrea Pitsch, Michael Tomasello.Dogs steal in the dark. Animal Cognition, 2012; DOI:10.1007/s10071-012-0579-6
University of Portsmouth (2013, February 11). Dogs may understand human point of view.ScienceDaily. Retrieved February 12, 2013, from http://www.sciencedaily.com/releases/2013/02/130211090840.htm

Most Comprehensive Tree of Life Shows Placental Mammal Diversity Exploded After Age of Dinosaurs

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Feb. 7, 2013 — An international team of scientists including University of Florida researchers has generated the most comprehensive tree of life to date on placental mammals, which are those bearing live young, including bats, rodents, whales and humans.

 

Appearing  February 7 in the journalScience, the study details how researchers used both genetic and physical traits to reconstruct the common ancestor of placental mammals, the creature that gave rise to many mammals alive today. The data show that contrary to a commonly held theory, the group diversified after the extinction of dinosaurs 65 million years ago. The research may help scientists better understand how mammals survived past climate change and how they may be impacted by future environmental conditions.

UF researchers led the team that analyzed the anatomy of living and fossil primates, including lemurs, monkeys and humans, as well as their closest living relatives, flying lemurs and tree shrews. The multi-year collaborative project was funded by the National Science Foundation Assembling the Tree of Life Program.

“With regards to evolution, it’s critical to understand the relationships of living and fossil mammals before asking questions about ‘how’ and ‘why,’ ” said co-author Jonathan Bloch, associate curator of vertebrate paleontology at the Florida Museum of Natural History on the UF campus. “This gives us a new perspective of how major change can influence the history of life, like the extinction of the dinosaurs — this was a major event in Earth’s history that potentially then results in setting the framework for the entire ordinal diversification of mammals, including our own very distant ancestors.”

Visual reconstruction of the placental ancestor — a small, insect-eating animal — was made possible with the help of a powerful cloud-based and publicly accessible database called MorphoBank. Unlike other reconstructions, the new study creates a clearer picture of the tree of life by combining two data types: Phenomic data includes observational traits such as anatomy and behavior, while genomic data is encoded by DNA.

“Discovering the tree of life is like piecing together a crime scene — it is a story that happened in the past that you can’t repeat,” said lead author Maureen O’Leary, an associate professor in the department of anatomical sciences in the School of Medicine at Stony Brook University and research associate at the American Museum of Natural History. “Just like with a crime scene, the new tools of DNA add important information, but so do other physical clues like a body or, in the scientific realm, fossils and anatomy. Combining all the evidence produces the most informed reconstruction of a past event.”

Researchers recorded observational traits for 86 placental mammal species, including 40 fossil species. The resulting database contains more than 12,000 images that correspond to more than 4,500 traits detailing characteristics like the presence or absence of wings, teeth and certain bones, type of hair cover and brain structures. The dataset is about 10 times larger than information used in previous studies of mammal relationships.

“It was a great way to learn anatomy, in a nutshell,” said co-author Zachary Randall, a UF biology graduate student and research associate at the Florida Museum. “While coding for humans, I could clearly see which anatomical features are unique, shared or not shared with other groups of mammals. This study is a great backbone for future work.”

Bloch and Randall collaborated with study co-authors Mary Silcox of the University of Toronto Scarborough and Eric Sargis of Yale University to characterize humans, plus seven other living and one fossil species from the clade Euarchonta, which includes primates, tree shrews and flying lemurs.

“I think this database is amazing because it’s being presented in such a way that it will be reproducible for the future generations,” Bloch said. “It illustrates exactly what we did and leaves nothing to the imagination — you can actually go to the pictures and see it.”

The evolutionary history of placental mammals has been interpreted in very different ways depending on the data analyzed. One leading analysis based on genomic data alone predicted that a number of placental mammal lineages existed in the Late Cretaceous and survived the Cretaceous-Paleogene extinction.

“It has been suggested that primates diverged from other mammals well before the extinction of the dinosaurs, but our work using direct evidence from the fossil record tells a different story,” Bloch said.

The team reconstructed the anatomy of the placental common ancestor by mapping traits most strongly supported by the data to determine it had a two-horned uterus, a brain with a convoluted cerebral cortex, and a placenta in which maternal blood came in close contact with membranes surrounding the fetus, as in humans.

Story Source:

The above story is reprinted from materials provided byUniversity of Florida.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

  1. M. A. O’Leary, J. I. Bloch, J. J. Flynn, T. J. Gaudin, A. Giallombardo, N. P. Giannini, S. L. Goldberg, B. P. Kraatz, Z.-X. Luo, J. Meng, X. Ni, M. J. Novacek, F. A. Perini, Z. S. Randall, G. W. Rougier, E. J. Sargis, M. T. Silcox, N. B. Simmons, M. Spaulding, P. M. Velazco, M. Weksler, J. R. Wible, A. L. Cirranello. The Placental Mammal Ancestor and the Post-K-Pg Radiation of Placentals. Science, 2013; 339 (6120): 662 DOI: 10.1126/science.1229237
University of Florida (2013, February 7). Most comprehensive tree of life shows placental mammal diversity exploded after age of dinosaurs. ScienceDaily. Retrieved February 10, 2013, from http://www.sciencedaily.com/releases/2013/02/130207141458.htm