Connexion
When the history of our ancestry is written, the fact that we've interbred with some of our closest relatives, the Neanderthals and Denisovans, will have a central role. And it will be tempting to write it as a very tidy story: Once we got genomes from these other groups, it was possible to identify the sequences in our genomes that we shared with them.
But in reality, as scientists started poking large enough collections of data, there were regular hints of some strange ancestry in our genomes. It was hard to pin down, though, at least in part because the 2 percent on average of Neanderthal DNA found in many populations does not guarantee that any two individuals will have the same 2 percent. So having the genomes of those two groups made sense of some things researchers had already been seeing.
But knowing what we do about Neanderthal and Denisovan DNA is now allowing researchers to answer a somewhat different question: Is there anything else? Using recently developed analytical techniques, they find evidence of a third lineage that we apparently interbred with before any modern humans left Africa. Again, there were hints of this earlier, but so far, there has been no genome from a modern human relative to help us understand the details—the source of this DNA remains a "ghost lineage."
A 63-year-old woman arrived at a hospital in pain. She had abruptly lost vision in her right eye. As doctors got to work, the series of findings that followed revealed not just bad news for her, but an alarming revelation of how infections can evolve within a patient.
The woman's remarkable case was reported this week in the New England Journal of Medicine. According to her doctors, initial magnetic resonance imaging scans of her brain quickly identified the key problem. The imaging indicated inflammation in her right eye, but also picked up lesions in her brain. Specifically, the imaging suggested problems in her parietal lobe, which is a hub for processing sensory information.
The doctors suspected she had a brain abscess. Laboratory results of her blood and urine samples suggested an infection as well as kidney problems. Doctors decided to open up her skull for a brain biopsy. Inside, the surgical team was met with pus. Meanwhile, doctors worked to identify the bacteria found in two urine samples.
Until about 60,000 years ago, diminutive hominin cousins, Homo floresiensis (affectionately nicknamed Hobbits for obvious reasons), shared the island of Flores with Komodo dragons, pygmy elephants, and giant rats.
Based on the presence of hominin and pygmy elephant bones in the same layers of cave sediment, it originally looked like the Hobbits had hunted and butchered dwarf elephants—an impressive feat for such a tiny hominin. But according to University of Tübingen anthropologist Elizabeth Veatch and her colleagues, it was the Komodo dragons that were the hunters, while the Hobbits only showed up to scavenge what was left.
If Veatch and her colleagues are right, their findings may challenge some of the assumptions we’ve made about Homo floresiensis—and about which hominin species was the first to venture into the wider world beyond Africa.
Did you know that the space shuttle once launched the Statue of Liberty into space?
In fact, there were two "Lady Liberties" on board Discovery when it lifted off on its fourth flight in April 1985. To be fair, each statue was only 15 inches tall (38.1 centimeters), but they were also each made of copper that was removed from the full-size statue during its then-still-ongoing restoration.
After the weeklong STS-51D mission was over, one of the space-flown statues was placed on display, and the other was melted down to create copper seals, which were then sold to the public by the Statue of Liberty Ellis Island Centennial Commission.
Tags de l’article
Understanding the origin of life requires addressing a collection of overlapping scientific questions. We've made a lot of progress toward explaining how simple chemicals present on an early Earth built the complex molecules used by life and how some of those chemicals built the first genetic/catalytic molecules. But we're much further from understanding a key conundrum: How did membranes end up surrounding the first cells?
It's relatively easy to make membranes spontaneously form in water, and they'll enclose anything dissolved in that water, including nucleic acids. But the membranes then cut their interior off from everything else in the solution. Any interesting chemical reactions enclosed there would eat through the raw materials and grind to a halt.
Now, a lab at the University of Minnesota has announced that it has developed a simplified system in which a membrane encloses some genetic material but can continually import new materials supplied to it. The system also spontaneously divides, producing a few generations of "offspring" before things start failing. It's still extremely dependent upon human intervention, but it might provide a new avenue to explore questions about the origin of life and what a truly minimalistic form of life might look like.
All the Homo naledi skeletons in Rising Star Cave are female, and that probably didn’t happen by accident.
In 2013, a team of anthropologists led by Lee Berger unearthed the remains of more than 20 small-bodied hominins (ancient relatives of humans), all 335,000 to 236,000 years old, from the Rising Star Cave System in South Africa. Excavations at Rising Star have sparked debate about whether these little hominins had all ended up in the caves by tragic accident, or whether they’d been carefully placed there by other members of their enigmatic species, dubbed Homo naledi.
Now there's a plot twist that may speak to how the remains got there: All of the hominins in Rising Star are female, at least according to the proteins in their dental enamel.
For decades, biologists thought that early tetrapods, ancient vertebrates that started conquering the land over 300 million years ago, developed like modern amphibians—beginning their lives as purely aquatic tadpoles and then metamorphosing into terrestrial adults. “A lot of that comes from this old ‘scala naturae’ idea that you had fish that evolved into the next stage up, which were amphibians, and then amphibians evolved into the next stage up, which were reptiles that evolved into birds and mammals,” said Jason Pardo, a research associate at the Field Museum.
We’ve never had evidence that early tetrapods had an amphibian lifestyle; we have assumed it because it made intuitive sense. “It’s easier to make the transition from water to land if you’re already making that transition as part of your life cycle,” Pardo said. But now, a new Science study that Pardo co-authored with Arjan Mann (the Field Museum's assistant curator of early tetrapods) shows our most basic assumptions about the first tetrapods that started living on land might be wrong.
The researchers' study focused mainly on embolomers, an extinct group of large predators that lived roughly 300 million years ago. Embolomers looked like a cross between a crocodile and an eel, with large skulls full of sharp teeth, followed by long, eel-like bodies. It had short, stocky limbs adapted mainly for paddling in water, but also capable of powering brief, clumsy excursions on land. They are thought to be one of the first vertebrates that made a partial transition from an aquatic to a terrestrial lifestyle. These animals could reach over three meters in length, but to understand the very beginning of their life cycle, scientists focused on examining some of their centimeter-scale babies.
Last week, we looked at a new study of the origin of complex cells, one that showed that our ancestors' genomes were pieced together from bits and pieces of multiple species. It put a spotlight on a phenomenon called horizontal gene transfer, in which a gene from one species is incorporated into the genome of a distantly related species. The frequency of horizontal gene transfer means that, in addition to the neatly branching trees that relate species by common descent, there are small threads connecting distant branches of the tree of life.
It's easy to see why horizontal gene transfer would be common among microbes. They often live in complex communities that are likely awash in the DNA of dead and damaged cells. Plus, bacteria and archaea lack a membrane between their DNA and the rest of the cell, making it easier for environmental DNA to find its way to the genome.
However, a new study this week shows that horizontal gene transfers are remarkably common even in multicellular animals. And it does so by examining the genomes of multiple cockroach species, which have had bits of bacterial DNA for millions of years.
We tend to view ourselves and the complex cells that build us as a distinct branch of the tree of life from the compact, seemingly featureless cells of bacteria and archaea. But we've found that our genome is actually a hybrid, a mish-mash of genes from bacteria and archaea, along with some that have evolved in our own lineage.
Scientists gradually settled on a simple explanation for this: the first complex cells were the product of a fusion between archaeal cells and bacteria, with the bacteria ultimately evolving into the mitochondria, a chemical-power-generating structure that still retains a bit of its own genome. Over time, many of the other bacterial genes were transferred to the nucleus of what was becoming what we now call a eukaryote, intermingling with the archaeal genes there.
But a new study has taken a careful look at some of the genes shared by all eukaryotes and comes to the conclusion that the reality is a little more complicated and that there were several waves of gene transfers from bacteria. The big picture of a merger between bacteria and archaea is still right, but it was only part of a picture where gene transfers among species were commonplace.
Humanity's ancestry has grown far clearer thanks to our ability to obtain ancient DNA. We now know that, as humans left Africa, they interbred with the groups they met there, Neanderthals and Denisovans. Evidence from the Denisovan genome also suggests that this was nothing new; the Denisovans had apparently interbred with an even earlier group. But the identity of that group remained a bit of a mystery.
Now, some evidence from ancient proteins suggests that the mystery group was Homo erectus, a species that left Africa over a million years ago and spread throughout Eurasia. And, thanks to the Denisovans, it appears that modern humans inherited some of that Homo erectus DNA.
Without access to all the repair enzymes made by living cells, DNA rapidly degrades. The double helix fragments, and bases change identity or fall off entirely. While cooler, drier environments slow this process, it sets a hard limit on how far back in time we can obtain DNA sequences. So far, it seems that Homo erectus remains on the far side of that time limit.
For decades, astronomers looking through telescopes like Hubble have been trying to catch a glimpse of the ancient epoch when the Universe's first generation of stars ignited. But the small galaxies that were the building blocks of the cosmos we know today were too faint to spot, even by the most powerful instruments. Now it seems astronomers finally have two things on their side: the Webb Space Telescope and a bit of luck.
In a recent paper in Nature, a team of scientists led by Kimihiko Nakajima, an astronomer at the Kanazawa University, Japan, used the James Webb Space Telescope to observe an ultra-faint galaxy called LAP1-B as it existed roughly 800 million years after the Big Bang. It’s the most chemically primitive galaxy we’ve ever seen.
The LAP1-B is 13 billion light-years away from Earth. To observe an object that faint and distant, even the huge, gold-coated beryllium mirrors of JWST were not enough on their own. We spotted it due to a massive cluster of galaxies called the MACS J046, which warps the spacetime between us and the LAP1-B.
The genetic code is central to life. With minor variations, everything uses the same sets of three DNA bases to encode the same 20 amino acids. We have discovered no major exceptions to this, leading researchers to conclude that this code probably dated back to the last common ancestor of all life on Earth. But there has been a lot of informed speculation about how that genetic code initially evolved.
Most hypotheses suggest that earlier forms of life had partial genetic codes and used fewer than 20 amino acids. To test these hypotheses, a team from Columbia and Harvard decided to see if they could get rid of one of the 20 currently in use. And, as a first attempt, they engineered a portion of the ribosome that worked without using an otherwise essential amino acid: isoleucine.
First off, why would you do this? Most work in the field has focused on altering the genetic code in ways that are useful, such as using more than 20 amino acids to enable interesting chemistry.
If you look at a Neanderthal skull and a Homo sapiens skull, they're visibly different: Neanderthal skulls are lower and longer, whereas ours tend to be rounder. However, those differences probably don’t say much about the brains within them, according to a recent study, which compared MRI scans of modern people’s brains with casts of the inside of Neanderthal skulls.
The results suggest that there’s more variation in brain size among modern people than between Neanderthals and Pleistocene Homo sapiens. And because brain size is actually a terrible way to predict cognitive capability, Neanderthals could have been a lot more like us than some previous studies have claimed, which definitely fits what the archaeological record tells us about how they lived. It would also mean that our species probably didn't out-compete the Neanderthals by being smarter or more adaptable.
Years after you die, the inner vault of your skull will hold the shape of your brain; if future archaeologists make a cast of that inner space, they’ll get a neat resin model of the outer contours of your brain, called an endocast. (Sediment that filled the skull of an Australopithecus africanus child who died 2.8 million years ago did this naturally, creating an endocast that’s half rocky brain-sculpture and half sparkling crystal.) For years, researchers have studied endocasts of Neanderthal skulls, trying to piece together how their brains were different or similar to ours. And that has been a matter of some debate.
Grâce à une datation précise par paléomagnétisme, une étude publiée dans Nature révèle que les fossiles de la grotte aux Hominidés de Casablanca datent de 773 000 ans. Cette découverte majeure comble un vide chronologique crucial et confirme l'enracinement africain des ancêtres d'Homo sapiens, au moment même où les lignées humaines divergeaient vers l'Europe et l'Asie.
Lateo.net - Flux RSS en pagaille (pour en ajouter : @ moi)