Adaptive radiations in the Mesozoic

March 22, 2018, Ludwig Maximilian University of Munich

The Ginglymodi Propterus elongatus in the Paleontological Museum Munich. Credit: A. López-Arbarello, LMU

Bony fishes are the most diverse of all extant vertebrate groups. A comprehensive phylogenetic analysis of the group now provides new insights into its 250-million-year evolutionary history. 

With some 30,000 currently recognized species, the true bony fishes or teleosts, which belong to the Neopterygii, account for more than half of modern vertebrate diversity.

In collaboration with her Argentinian colleague Emilia Sferco, LMU paleontologist Adriana López-Arbarello, who is in the Paleontology and Geobiology Section at the Department of Earth and Environmental Sciences and is also a member of the Geobiocenter, has undertaken a new analysis of the group's family tree. Their results, which shed new light on 250 million years of evolutionary history, appear in the online journal Royal Society Open Science.

More than 99% of the species of Neopterygii in the world today belong to a single lineage, the true bony fishes or Teleostei. Relative to the rich diversity of the teleosts, the other two neopterygian lineages are strikingly impoverished. The Ginglymodi are made up of the seven recognized species of gars, and the Halecomorphi ¬now consist of a single species, Amia calva, commonly known as the bowfin. In their heyday in the Mesozoic – the Age of the Dinosaurs – the picture was very different. At that time, the Ginglymodi and the Halecomorphi were highly diverse, and they dominated the oceans and freshwater habitats, while the teleosts were a much more modest presence in the world's waters.

The huge difference in the numbers of extant species assigned to the three groups makes it extremely difficult to unravel the relationships between them. However, fossil specimens can help to correct this imbalance, and thus make it possible to elucidate their 250-million-year history. "In fact," says Adriana López-Arbarello, "our study was motivated by the discovery of one very unusual fossil genus." Specimens of this genus, dated to 240 million years ago, which exhibit traits characteristic of both the Ginglymodi and the Halecomorphi, were recovered by a Swiss team from an important marine fossil Lagerstätte in the Alps. Their discoverers invited López-Arbarello to participate in the anatomical investigation and description of their finds.

Later, López-Arbarello und Sferco took up the task of working out how the new genus fits into the neopterygian family tree. To do so, they assembled the largest morphological dataset yet constructed for fossils of this subclass.

Phylogenetic analysis of these data revealed that Ginglymodi and Halecomorphi are more closely related to each other than either is to Teleostei. This confirms what has been regarded by many paleontologists as a controversial notion – namely that the first two taxonomic groups share a common ancestor, and can therefore be subsumed into a single common taxon. In addition, the new study shows that, in the Mesozoic, all three lineages underwent several rapid adaptive radiations, i.e. during which large numbers of novel species evolved over comparatively short periods of time. Not only were Ginglymodi and Halecomorphi represented by enormous numbers of species, these species were characterized by a broad spectrum of morphological and ecological specializations. Indeed, some of the most impressive Mesozoic fossils belong to these lineages. One example is Scheenstia maximus, which grew to a length of nearly 2 meters.

The Teleostei subsequently developed a wide variety of feeding habits, and enhanced their maneuverability and the efficiency of their reproductive strategies, while the diversity of the other two lineages progressively declined. Precisely why this happened is still unclear. "One can perhaps compare this situation with the evolution of the dinosaurs," López-Arbarello remarks. "Many successful and diverse groups of dinosaurs died out at the end of the Mesozoic. Only one survived, and its diversification continues – the birds. We have no really convincing explanation for their success either. – So we still have a lot to learn about the fauna of the Mesozoic and the biological world of which it was a part."

Explore further: First-known ginglymodian fish found from the middle triassic of Eastern Yunnan Province, China

 

More information: Adriana López-Arbarello et al. Neopterygian phylogeny: the merger assay, Royal Society Open Science (2018). DOI: 10.1098/rsos.172337

New study suggests that last common ancestor of humans and apes was smaller than thought

 Novo estudo sugere que o último ancestral comum de humanos e macacos foi menor que o pensamento

 

October 12, 2017, American Museum of Natural History

New research suggests that the last common ancestor of apes—including great apes and humans—was much smaller than previously thought, about the size of a gibbon. The findings, published today in the journal Nature Communications, are fundamental to understanding the evolution of the human family tree.

"Body size directly affects how an animal relates to its environment, and no trait has a wider range of biological implications," said lead author Mark Grabowski, a visiting assistant professor at the Eberhard Karls University of Tübingen in Germany who conducted the work while he was a postdoctoral fellow in the American Museum of Natural History's Division of Anthropology. "However, little is known about the size of the last common ancestor of humans and all living apes. This omission is startling because numerous paleobiological hypotheses depend on body size estimates at and prior to the root of our lineage."

Among living primates, humans are most closely related to apes, which include the lesser apes (gibbons) and the great apes (chimpanzees, gorillas, and orangutans). These "hominoids" emerged and diversified during the Miocene, between about 23 million to 5 million years ago. Because fossils are so scarce, researchers do not know what the last common ancestors of living apes and humans looked like or where they originated.

To get a better idea of body mass evolution within this part of the primate family tree, Grabowski and coauthor William Jungers from Stony Brook University compared body size data from modern primates, including humans, to recently published estimates for fossil hominins and a wide sample of fossil primates including Miocene apes from Africa, Europe, and Asia. They found that the common ancestor of apes was likely small, probably weighing about 12 pounds, which goes against previous suggestions of a chimpanzee-sized, chimpanzee-like ancestor.

Among other things, the finding has implications for a behavior that's essential for large, tree-dwelling primates: it implies that "suspensory locomotion," overhand hanging and swinging, arose for other reasons than the animal simply getting too big to walk on top of branches. The researchers suggest that the ancestor was already somewhat suspensory, and larger body size evolved later, with both adaptations occurring at separate points.

The development of suspensory locomotion could have been part of an "arms race" with a growing number of monkey species, the researchers said. Branch swinging allows an animal to get to a prized and otherwise inaccessible food—fruit on the edges of foliage—and larger body would let them engage in direct confrontation with monkeys when required.
The new research also reveals that australopiths, a group of early human relatives, were actually on average smaller than their ancestors, and that this smaller size continued until the arrival of Homo erectus.

"There appears to be a decrease in overall body size within our lineage, rather than size simply staying the same or getting bigger with time, which goes against how we generally think about evolution," Grabowski said.

Explore further: Human ancestor was less-chimp-like than thought: study

More information: Mark Grabowski et al, Evidence of a chimpanzee-sized ancestor of humans but a gibbon-sized ancestor of apes, Nature Communications (2017). DOI: 10.1038/s41467-017-00997-4

New life form answers question about evolution of cells

March 19, 2018, University of Groningen

The left panel shows: EM image of a normal E. coli cell. Right panel: an engineered cell with a mixed membrane, which shows an elongated form. Credit: Photo's University of Wageningen / Van der Oost laboratory

Bacteria and Archaea are two of the three domains of life. Both must have evolved from the putative last universal common ancestor (LUCA). One hypothesis is that this happened because the cell membrane in LUCA was an unstable mixture of lipids. Now, scientists from the University of Groningen and Wageningen University have created such a life form with a mixed membrane and discovered it is, in fact, stable, refuting this hypothesis. The results will be published in the journal Proceedings of the National Academy of Sciences in the week of 19 March.

There are many ideas on how cellular life could have evolved billions of years ago. Protocells may have formed in clay minerals, or as simple lipid vesicles. In the latter scenario, something called the 'lipid divide' would have occurred, creating the separate domains of bacteria and archaea, explains University of Groningen Professor of Molecular Microbiology Arnold Driessen. "The lipid membranes of both domains are different, composed of phospholipids that are each other's mirror image."

Mixed membranes

In technical terms, the lipids in the membranes of bacteria are made up of straight-chain fatty acids that are ester-linked to a backbone of glycerol-3-phosphate. But the lipids in the archaeal membrane have a backbone of glycerol-1-phosphate, to which isoprenoids are linked by ether bonds.

The idea behind the lipid divide is that a common ancestor of both bacteria and archaea had a cell membrane in which both types of lipid were mixed. "This mixed membrane would be less stable than a homogenous membrane of just one type of phospholipid, so eventually a split occurred, resulting in the two domains of Bacteria and Archaea," says Driessen.

All this would have happened over 3.5 billion years ago, so hard evidence is lacking. Driessen and his co-workers therefore decided to "reverse-engineer" a micro-organism with a mixed membrane. "This has been tried before, but these experiments resulted in bacteria with only very small amounts (< 1 percent) of archaeal lipids."

Left panel: EM image of a normal dividing E. coli cell. Right panel: an engineered cell with high archaeal lipid production, showing lobular irregularities in the cell membrane. Credit: Photo's University of Wageningen / Van der Oost laboratory

However, in the new study, this increased to 30 percent. Two key breakthroughs made this possible: "In previous research we discovered an enzyme crucial to the production of archaeal membrane lipids. This takes three steps, and before that, only two of the enzymes involved were known." The other breakthrough came from scientists at Wageningen University, explains Driessen. "They managed to increase the production of isoprenoids in the bacterium E. coli." Isoprenoids are a ubiquitous class of organic compounds which includes many natural flavours, colours or fragrances.

Both discoveries were transferred to a normal E. coli bacterium, which was a major feat of genetic engineering.

"And we didn't know if the end result would be a viable cell," says Driessen. But in the end, it worked out well. With some fine tuning, the scientists created a cell in which all phosphatidylglycerol, the lipids forming the basic bilayer of the bacterial membrane, were replaced by their archaeal equivalent (archaetidylglycerol).

This accounts for 30 percent of the lipids in the membrane. "This bacterium grew at normal speed and was stable," says Driessen. "So this result does not support the hypothesis that a mixed membrane is inherently instable and could thus have created the lipid divide."

Driessen notes that the archaeal enzymes for membrane lipid production are less specific in the reactions they catalyze than their bacterial equivalents. "They appear to be more "primordial." So the evolution of enzyme specificity could have been a driver for the divide." There is, of course, one major caveat—the experiments were done in modern E. coli bacteria, which have evolved 3.5 billion years beyond the original split with the archaea.

"The robustness of these mixed cells surprised us. We expected more problems keeping them alive. After all, what we have engineered does amount to a new life form." The engineered cells were more elongated than the original E. coli. And when the production of archaeal lipids was very high, growth slowed and the membrane developed lobular appendages.

Apart from the evolutionary implications, this discovery could spawn new research: "For example, we could engineer a bacterial expression system for archaeal membrane proteins, such as those produced by hyperthermophiles that grow at extremely high temperatures and pressure."
The work by the two Dutch universities is part of the Origins Center, a national programme dedicated to research into the origin of life on our planet.

Explore further: Formation of cell membrane component domains in artificial lipid bilayer

 

More information: Antonella Caforio el al., "Converting Escherichia coli into an archaebacterium with a hybrid heterochiral membrane," PNAS (2018). www.pnas.org/cgi/doi/10.1073/pnas.1721604115
 

Journal reference: Proceedings of the National Academy of Sciences
Provided by: University of Groningen

Study of 385-million-year-old shark suggests humans and sharks shared common ancestor 440 million years ago

January 4, 2018 by Bob Yirka, Phys.org report

Credit: CC0 Public Domain

A team of researchers with the University of Chicago, University College Dublin and Cambridge University studying a 385-million-year-old shark fossil has found evidence that suggests humans and sharks shared a common ancestor approximately 440 million years ago. 

The researchers were studying a shark specimen found in Germany back in 2001. At the time, it was believed the shark was toothless, and for that reason, scientists gave it the name Gladbachus adentatus. In this new effort, the researchers conducted a much more thorough study of the remains, and in so doing, discovered that it represented a transitional species between acanthodians and chondrichthyes. This bit of evidence offered a better picture of a time period for which there are few fossil records. It suggests a new estimate for the time during which humans and sharks shared a common ancestor—approximately 440 million years ago.

The specimen is the only one of its kind ever found—that of a shark that lived approximately 385 million years ago, during a time period known as the Devonian, which lasted from 416 million to 358 million years ago. The remains consisted of three sections, all compressed flat in resin. The resin casing preserved much of the endoskeleton, which allowed the team to collect tissue samples. Also preserved were teeth, a cranium, cartilage and gill details. The team studied all of the parts using CT scans, which gave them a more complete picture of what the creature once looked like. The researchers note that the body of the specimen looked like a sheet of scales, and that the bones in its head were very coarse.

The researchers also note that even as study of the specimen has clarified some of the evolutionary history of sharks, it has also complicated understanding of their lineage—they found evidence suggesting that shark evolution has many branches, several of which appear to have converged, leading to characteristics found in modern sharks such as a long throat and multiple gill slits. Their study also confirmed that G. adentatus actually had an abundance of teeth, both small and large.

Explore further: Fossil ancestor shows sharks have a bony past

 

More information: Michael I. Coates et al. An early chondrichthyan and the evolutionary assembly of a shark body plan, Proceedings of the Royal Society B: Biological Sciences (2018). DOI: 10.1098/rspb.2017.2418

Abstract
 
Although relationships among the major groups of living gnathostomes are well established, the relatedness of early jawed vertebrates to modern clades is intensely debated. Here, we provide a new description of Gladbachus, a Middle Devonian (Givetian approx. 385-million-year-old) stem chondrichthyan from Germany, and one of the very few early chondrichthyans in which substantial portions of the endoskeleton are preserved. Tomographic and histological techniques reveal new details of the gill skeleton, hyoid arch and jaws, neurocranium, cartilage, scales and teeth.

Despite many features resembling placoderm or osteichthyan conditions, phylogenetic analysis confirms Gladbachus as a stem chondrichthyan and corroborates hypotheses that all acanthodians are stem chondrichthyans. The unfamiliar character combination displayed by Gladbachus, alongside conditions observed in acanthodians, implies that pre-Devonian stem chondrichthyans are severely under-sampled and strongly supports indications from isolated scales that the gnathostome crown group originated at the latest by the early Silurian (approx. 440 Ma). Moreover, phylogenetic results highlight the likely convergent evolution of conventional chondrichthyan conditions among earliest members of this primary gnathostome division, while skeletal morphology points towards the likely suspension feeding habits of Gladbachus, suggesting a functional origin of the gill slit condition characteristic of the vast majority of living and fossil chondrichthyans.

Journal reference: Proceedings of the Royal Society B

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Human breeding practices found to be likely reason for lack of paternal DNA diversity in modern horses

Práticas de reprodução humana são provavelmente a razão para a falta de diversidade de DNA paterno em cavalos modernos

April 19, 2018 by Bob Yirka, Phys.org report

Credit: CC0 Public Domain

A team of researchers from across Europe has found that human breeding practices, particularly during the Iron Age, are likely the cause of a lack of variability in paternally inherited DNA in modern horses. In their paper published on the open access site Science Advances, the group describes the DNA study they undertook to better understand the genetic history of the modern horse and what they found. 

It has been known for some time that modern horses are genetically diverse, maternally speaking, but have a distinct lack of variability on the paternal side. The prevailing theory regarding this state of affairs is that during the time when horses were first being domesticated there were many more mares than stallions. In this new effort, the researchers suggest a different reason—they believe it is due to selective breeding, particularly during the Iron Age. The group came to this conclusion after conducting an extensive DNA analysis of both modern and ancient horses.

The study consisted of gathering DNA samples from 350 horses covering the past 5,000 years and attempting to genotype them—in the end they were able to genotype just 86 of the samples, but that was enough to provide the team with a means for creating a timeline for four major haplotypes: Y-HT-1 through 4. Y-HT-1, the team notes, is dominant in modern horses. 

To create the timeline, the team divided the DNA samples into four time periods: earlier than 2,200 BC, between 2,200 BC and 900 BC, between 900 BC to 400 AD and from 400 AD till today. In looking at the haplotypes, the researchers found all but Y-HT-1 were heavily represented in the first period, but over time Y-HT-1 became increasingly prominent as the other three became less so.

The researchers note that Y-HT-1 began its progression to dominance during the Bronze Age, a time during which humans were migrating across Eurasia. But the Iron Age was when Y-HT-1 really began to assert itself, a period during which the Roman Empire was reaching across Europe and changing ideas about how horses should be bred by focusing on males instead of females. This, the researchers claim, suggests that the lack of paternal diversity is very likely due to human breeding practices.

Explore further: Most modern horses are descendants of recently imported oriental stallions

More information: Saskia Wutke et al. Decline of genetic diversity in ancient domestic stallions in Europe, Science Advances (2018). DOI: 10.1126/sciadv.aap9691

Abstract
 
Present-day domestic horses are immensely diverse in their maternally inherited mitochondrial DNA, yet they show very little variation on their paternally inherited Y chromosome. Although it has recently been shown that Y chromosomal diversity in domestic horses was higher at least until the Iron Age, when and why this diversity disappeared remain controversial questions. We genotyped 16 recently discovered Y chromosomal single-nucleotide polymorphisms in 96 ancient Eurasian stallions spanning the early domestication stages (Copper and Bronze Age) to the Middle Ages.

Using this Y chromosomal time series, which covers nearly the entire history of horse domestication, we reveal how Y chromosomal diversity changed over time. Our results also show that the lack of multiple stallion lineages in the extant domestic population is caused by neither a founder effect nor random demographic effects but instead is the result of artificial selection—initially during the Iron Age by nomadic people from the Eurasian steppes and later during the Roman period. Moreover, the modern domestic haplotype probably derived from another, already advantageous, haplotype, most likely after the beginning of the domestication. In line with recent findings indicating that the Przewalski and domestic horse lineages remained connected by gene flow after they diverged about 45,000 years ago, we present evidence for Y chromosomal introgression of Przewalski horses into the gene pool of European domestic horses at least until medieval times.

Journal reference: Science Advances

Ancient DNA offers new view on saber-toothed cats' past

DNA antigo oferece nova visão sobre o passado de felinos com dentes de sabre

October 19, 2017, Cell Press

Drawing of a Homotherium. Credit: Binia De Cahsan

Researchers who've analyzed the complete mitochondrial genomes from ancient samples representing two species of saber-toothed cats have a new take on the animals' history over the last 50,000 years.

The data suggest that the saber-toothed cats shared a common ancestor with all living cat-like species about 20 million years ago. The two saber-toothed cat species under study diverged from each other about 18 million years ago.

"It's quite crazy that, in terms of their mitochondrial DNA, these two saber-toothed cats are more distant from each other than tigers are from house cats," says Johanna Paijmans at the University of Potsdam in Germany.

Paijmans and colleagues reconstructed the mitochondrial genomes from ancient-DNA samples representing three Homotherium from Europe and North America and one Smilodon specimen from South America. One of the Homotherium specimens under investigation is a unique fossil: a 28,000-year-old mandible recovered from the North Sea.

"This find was so special because Homotherium is generally believed to have gone extinct in Europe around 300,000 years ago, so [this specimen is] over 200,000 years younger than the next-to-youngest Homotherium find in Europe," Paijmans explains.

The new DNA evidence confirmed that this surprisingly young specimen did indeed belong to a Homotherium. The discovery suggests that the saber-toothed cats continued to live in Europe much more recently than scientists previously thought.

Photograph of a homotherium fossil recovered from the North Sea. Credit: Natural History Museum Rotterdam

"When the first anatomically modern humans migrated to Europe, there may have been a saber-toothed cat waiting for them," Paijmans says.
The finding raises new questions about how and why the saber-toothed cats went extinct. Paijmans says they are now interested in studying DNA from other samples of saber-toothed cats. Although it will be technically challenging, they also hope to recover and analyze DNA from much older Homotherium specimens.

Photograph of a homotherium fossil recovered from the North Sea. Credit: Natural History Museum Rotterdam

Explore further: Saber-toothed kittens may have been born with thicker bones than other contemporary cats

More information: Current Biology, Paijmans et al.: "Evolutionary History of Saber-Toothed Cats Based on Ancient Mitogenomics" http://www.cell.com/current-biology/fulltext/S0960-9822(17)31198-3 , DOI: 10.1016/j.cub.2017.09.033
 

Journal reference: Current Biology
Provided by: Cell Press

Neanderthal nose: All the better to breathe with

April 4, 2018

Neanderthals are thought to have needed up to 4,480 calories a day to keep them alive in the European winter—some of their skulls have been on display as part of a Neanderthal exhibition at the Musee de l'Homme in Paris since last month

Neanderthals had large, protruding noses to warm and humidify cold, dry air, a study into the distinct design of our extinct European cousin's face suggested Wednesday. 

Using 3-D models of the skulls of Neanderthals, modern humans, and Homo heidelbergensis—considered to have been the common ancestor of both—an international research team found very different breathing adaptations.

Computerised "fluid dynamics" revealed that the shape of Neanderthal and human faces "condition air more efficiently" than H. heidelbergensis, suggesting that "both evolved to better withstand cold and/or dry climates," the researchers wrote in the journal Proceedings of the Royal Society B.
Neanderthals could also move "considerably more" air through their nasal cavity than could H. heidelbergensis or modern humans—possibly in response to higher energy requirements for their stocky bodies and hunting lifestyle.

Neanderthals were thought to have required as much as 4,480 calories per day to keep them alive in the European winter. For a modern human male, 2,500 daily calories are recommended.
A high-calorie intake requires more oxygen to burn the sugars, fats and proteins in our cells to produce energy.

Take a deep breath

Scientists have long debated over the reason for the Neanderthal's face shape, which includes a large, wide nose and protruding upper jaw.

One theory was they were built to exert more bite force.
But Wednesday's study said this was not the case. Computer simulations showed that Neanderthals "were not particularly strong biters" compared to humans.

Image illustrates the difference in skull and nose shape in the three human species tested. Airflow is color-coded for temperature (warmer colors = warmer air, cooler colors = colder air). Lines indicate that Neanderthal and modern-humans …more

But "where the Neanderthal really excelled is in its ability to move large volumes of air through its nasal passage, indicating a very high-energy lifestyle."

The conclusion, said the team, was "that the distinctive facial morphology of Neanderthals has been driven, at least in part, by adaptation to cold"—both to "condition" cold, dry air, and to absorb more oxygen.

Airflow comparison between a modern human (left) and Neanderthal (right) when breathing in air at 0°C (32°F), with warmer colors representing warmer air flow and cooler colors representing colder air flow. Skulls have been cut down the …more

Neanderthals emerged in Europe, Central Asia and the Middle East some 200,000 years ago. They vanished about 30,000 years ago—coinciding with the arrival of modern humans out of Africa.
The two groups briefly overlapped and interbred, and today, non-African people are said to carry about 1.5-2.1 percent of Neanderthal DNA.

Long portrayed as knuckle-dragging brutes, recent studies have started to paint a picture of Neanderthals as sophisticated beings who made art, took care of the elderly, buried their dead, and may have been the first jewellers—though they were probably also cannibals.

Explore further: Cold temperatures found to cause nasal structure similarities between Neanderthal and modern humans

 

More information: Computer simulations show that Neanderthal facial morphology represents adaptation to cold and high energy demands, but not heavy biting, Proceedings of the Royal Society B, rspb.royalsocietypublishing.or … .1098/rspb.2018.0085
 

Journal reference: Proceedings of the Royal Society B

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How to hunt a giant sloth—according to ancient human footprints

April 26, 2018 by Matthew Robert Bennett, Katie Thompson And Sally Christine Reynolds, The Conversation

Credit: Alex McClelland, Bournemouth University

Rearing on its hind legs, the giant ground sloth would have been a formidable prey for anyone, let alone humans without modern weapons. Tightly muscled, angry and swinging its fore legs tipped with wolverine-like claws, it would have been able to defend itself effectively. Our ancestors used misdirection to gain the upper hand in close-quarter combat with this deadly creature.

What is perhaps even more remarkable is that we can read this story from the 10,000-year-old footprints that these combatants left behind, as revealed by our new research published in Science Advances.

Numerous large animals such as the giant ground sloth – so-called megafauna – became extinct at the end of the Ice Age. We don't know if hunting was the cause but the new footprint evidence tells us how human hunters tackled such fearsome animals and clearly shows that they did.

These footprints were found at White Sands National Monument in New Mexico, US, on part of the monument that used by the military. The White Sands Missile Range, located close to the Trinity nuclear site, is famous as the birth place of the US space programme, of Ronald Reagan's Star Wars initiative and of countless missile tests. It is now a place where long-range rather than close-quarter combat is fine-tuned.
It is a beautiful place, home to a huge salt playa (dry lake) known as Alkali Flat and the world's largest gypsum dune field, made famous by numerous films including Transformers and the Book of Eli. At the height of the Ice Age it was home to a large lake (palaeo Lake Otero).

White Sands National Monument. Credit: Matthew Bennett, Bournemouth University, Author provided

As the climate warmed, the lake shrank and its bed was eroded by the wind to create the dunes and leave salt flats that periodically pooled water. The Ice Age megafauna left tracks on these flats, as did the humans that hunted them. The tracks are remarkable in that they are only a few centimetres beneath the surface and yet have been preserved for over 10,000 years.
Here there are tracks of extinct giant ground sloth, of mastodon, mammoth, camel and dire wolf. These tracks are colloquially known as "ghost tracks" as they are only visible at the surface during specific weather conditions, when the salt crusts are not too thick and the ground not too wet. Careful excavation is possible in the right conditions and reveals some amazing features.

Perhaps the coolest of these is a series of human tracks that we found within the sloth prints. In our paper, produced with a large number of colleagues, we suggest that the humans stepped into the sloth prints as they stalked them for the kill. We have also identified large "flailing circles" that record the sloth rising up on its hind legs and swinging its fore legs, presumably in a defensive, sweeping motion to keep the hunters at bay. As it overbalanced, it put its knuckles and claws down to steady itself.

Tracking the footprints. Credit: Matthew Bennett, Bournemouth University, Author provided

These circles are always accompanied by human tracks. Over a wide area, we see that where there are no human tracks, the sloth walk in straight lines. Where human track are present, the sloth trackways show sudden changes in direction suggesting the sloth was trying to evade its hunters.

Piecing together the puzzle, we can see how sloth were kept on the flat playa by a horde of people who left tracks along the its edge. The animals was then distracted by one stalking hunter, while another crept forward and tried to strike the killing blow. It is a story of life and death, written in mud.
What would convince our ancestors to engage is such a deadly game? Surely the bigger the prey, the greater the risk? Maybe it was because a big kill could fill many stomachs without waste, or maybe it was pure human bravado.

Footprint comparison. Credit: David Bustos, National Park Service

At this time at the end of the last Ice Age, the Americas were being colonised by humans spreading out over the prairie plains. It was also a time of animal extinctions. Many palaeontologists favour the argument that human over-hunting drove this wave of extinction and for some it has become an emblem of early human impact on the environment. Others argue that climate change was the true cause and our species is innocent.

It is a giant crime scene in which footprints now play a part. Our data confirms that human hunters were attacking megafauna and were practiced at it. Unfortunately, it doesn't cast light on the impact of that hunting. Whether humans were the ultimate or immediate cause of the extinction is still not clear. There are many variables including rapid environmental change to be considered. But what is clear from tracks at White Sands is that humans were then, as now, "apex predators" at the top of the food chain.

Plaster cast footprints. Credit: David Bustos, National Park Service

Explore further: Researcher IDs Ice Age sloth bone in Colorado

More information: David Bustos et al. Footprints preserve terminal Pleistocene hunt? Human-sloth interactions in North America, Science Advances (2018). DOI: 10.1126/sciadv.aar7621

Journal reference: Science Advances
Provided by: The Conversation

World's most venomous spiders are actually cousins

February 15, 2018, San Diego State University

A species of Australian funnel-web spider. Credit: Marshal Hedin

Two groups of highly venomous spiders might be seeing more of each other at family reunions. A new study led by San Diego State University biologist Marshal Hedin has found that two lineages of dangerous arachnids found in Australia—long classified as distantly related in the official taxonomy—are, in fact, relatively close cousins. The findings could help in the development of novel antivenoms, as well as point to new forms of insecticides.

The spiders in question are those from the families Atracinae and Actinopodidae and include Australian funnel-web spiders and eastern Australian mouse spiders, respectively. One member of Atracinae, Atrax robustus, is considered by many to be the most venomous spider in the world.

"A reasonable number of people get bitten every year, but basically nobody dies from it anymore because of the wide availability of antivenom," Hedin said.

Historically, the spiders were thought to have diverged from a common ancestor more than 200 million years ago and therefore were only distantly related. Based on their anatomy and other traits, funnel-web spiders and mouse spiders closely resemble other species of spiders known to be distantly related. Yet based on their highly similar venom—the same antivenom can treat bites from both Atricinae and Actinopodidae —many biologists suspected these spider groups might be more closely related than previously thought.

"The funnel-webs always were an uncomfortable fit in their taxonomic place," Hedin said. "I could see the writing on the wall."

So Hedin and colleagues, with help from biologists in New Zealand and Argentina, collected new spiders from both branches throughout Australia, sought out museum specimens and raided his own collection to come up with dozens of specimens representing various branches of spiders both closely and distantly related. Then the scientists sequenced large chunks of the spiders' genomes, looking for genetic patterns that would reveal how the species are related to one another.

After this analysis, the researchers discovered that the Australian funnel-web spiders and mouse spiders were, in fact, fairly closely related, although it's unclear exactly when they diverged from a common ancestor. In addition to solving that mystery, Hedin and colleagues discovered the existence of three entirely new taxonomic families of spiders. The researchers published their findings last month in Nature Scientific Reports.

Online taxonomy databases have already begun updating to reflect these changes, Hedin said. "We've convincingly resolved this relationship."

Knowing these spiders' ancestry could help scientists devise a kind of general-purpose antivenom to treat bites from a wide variety of related spider species, Hedin explained. In addition, funnel-web and mouse spider venom is notable for containing many different types of peptide molecules, including some that specifically target insects. Knowing more about how their venom evolved could help bioengineers to design bio-insecticides that target insects but are harmless to vertebrate animals.

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More information: Marshal Hedin et al. Phylogenomic reclassification of the world's most venomous spiders (Mygalomorphae, Atracinae), with implications for venom evolution, Scientific Reports (2018). DOI: 10.1038/s41598-018-19946-2
 

Journal reference: Scientific Reports
Provided by: San Diego State University