Affichage des articles dont le libellé est Évolution. Afficher tous les articles
Affichage des articles dont le libellé est Évolution. Afficher tous les articles
Flightless ancestor shows brain evolution in pterosaurs and birds took different paths
Flight is a rare skill in the animal world. Among vertebrates, it evolved only three times: in bats, birds, and the long-extinct pterosaurs. Pterosaurs were the pioneers, taking to the skies more than 220 million years ago, long before early bird relatives such as Archaeopteryx appeared, around 150 million years ago. While scientists have a detailed fossil record that sheds light on how birds' brains evolved for flight, the same story for pterosaurs has been far less clear. Until now.
In a new study published in Current Biology, an international team now reveals how pterosaurs evolved the neurological structures required for powered flight.
"The breakthrough was the discovery of an ancient pterosaur relative, a small lagerpetid archosaur named Ixalerpeton from 233-million-year-old Triassic rocks in Brazil," said Mario Bronzati, an Alexander von Humboldt fellow at the University of Tubingen in Germany and lead author of the study.
"We've had abundant information about early birds and knew they inherited their basic brain layout from their theropod dinosaur ancestors," added co-author Lawrence Witmer, professor of anatomy at the Ohio University Heritage College of Osteopathic Medicine. "But pterosaur brains seemed to appear out of nowhere. Now, with our first glimpse of an early pterosaur relative, we see that pterosaurs essentially built their own 'flight computers' from scratch."
How researchers mapped brain evolution
To piece together this evolutionary story, the researchers used high-resolution 3D imaging techniques, including microCT scanning, to reconstruct brain shapes from more than three dozen species. These included pterosaurs, their close relatives like Ixalerpeton, early dinosaurs and bird precursors, modern crocodiles and birds, and a wide range of Triassic archosaurs, the larger group that includes all these animals.
"Then, using statistical analysis of the size and 3D shape of their cranial endocasts, we were able to map the stepwise changes in brain anatomy that accompanied the evolution of flight," said co-author Akinobu Watanabe, associate professor of anatomy at the New York Institute of Technology College of Osteopathic Medicine.
Flight is a physiologically demanding form of locomotion and has long been assumed to require major neurological adaptations including enlargement of the brain to coordinate the complicated sensory and motor information required for powered flight. Previous studies of pterosaur brain structure had shown that they indeed shared some neurological similarities with bird precursors like Archaeopteryx, such as some enlargement of brain regions like the cerebrum and cerebellum involved with sensorimotor integration, as well as enlargement of visual centers like the optic lobes.
Ixalerpeton, the lagerpetid close relative of pterosaurs showed some but not all neurological traits of pterosaurs. For example, as Bronzati notes, "lagerpetids were probably tree-dwellers, and their brains already show features linked to improved vision, such as an enlarged optic lobe, an adaptation that may have later helped their pterosaur relatives take to the skies, but they still lacked key neurological traits of pterosaurs."
Lagerpetids like Ixalerpeton had brains intermediate in shape between more primitive archosaurs and pterosaurs but retain greater similarity to early dinosaurs. Other than the enlarged optic lobe that occupies a position in the brain similar to that in pterosaurs and birds and their close theropod relatives, there is little in Ixalerpeton that indicates what was to come in pterosaurs.
A unique feature of the brain of pterosaurs is a greatly enlarged flocculus, a structure of the cerebellum likely involved in processing sensory information from their membranous wings to keep their eyes fixed on a target while in flight. The flocculus in Ixalerpeton wasn't expanded like pterosaurs, instead resembling the modest flocculus of other archosaurs, including early birds and their close nonavian theropod relatives.
Likewise, the new analyses show that pterosaurs retained modest brain sizes.
Comparing pterosaur and bird brains
"While there are some similarities between pterosaurs and birds, their brains were actually quite different, especially in size," said co-author Matteo Fabbri, assistant professor of Functional Anatomy and Evolution at the Johns Hopkins University School of Medicine. "Pterosaurs had much smaller brains than birds, which shows that you may not need a big brain to fly."
Surprisingly, the overall brain shape of pterosaurs most closely resembled that of small, bird-like dinosaurs such as troodontids and dromaeosaurids, animals that had little or no powered flight ability. Yet pterosaurs and birds still represent two entirely independent experiments in the evolution of flight. Birds inherited a brain already adapted from their non-flying dinosaur ancestors, while pterosaurs evolved their flight-ready brains at the same time they developed their wings.
Birds' notably large brains, the authors note, likely came later and were tied more to increasing intelligence and complex behaviors rather than the act of flying itself. A key takeaway from the study is that, according to Witmer, "it apparently doesn't take a large brain to get into the air, and the later brain expansion in both birds and pterosaurs was likely more about enhancing cognition than about flying itself."
Another important takeaway is that paleontological fieldwork remains an engine for new breakthroughs.
"Discoveries from southern Brazil have given us remarkable new insights into the origins of major animal groups like dinosaurs and pterosaurs," co-author Rodrigo Temp Müller, a paleontologist at Universidade Federal de Santa Maria, Brazil, noted. "With every new fossil and study, we're getting a clearer picture of what the early relatives of these groups were like, something that would have been almost unimaginable just a few years ago."
Les scientifiques ont peut-être trouvé le plan du corps humain… au fond de l’océan !
Lorsque l’on pense à l’évolution du corps humain, on imagine souvent un cheminement complexe partant d’organismes relativement avancés, dotés de cerveaux et de systèmes nerveux sophistiqués. Pourtant, une découverte récente vient bouleverser cette vision en suggérant que certains des mécanismes fondamentaux à l’origine de notre organisation corporelle pourraient puiser leurs racines dans des créatures bien plus simples et éloignées de nous : les anémones de mer.
Ces organismes marins, membres de l’embranchement des cnidaires (qui comprend aussi les méduses et les coraux), sont loin d’être nos proches parents. Ils n’ont ni cerveau, ni système nerveux central, et leur corps est organisé de manière radiale, autour d’un point central, à l’inverse de la symétrie bilatérale qui caractérise les humains et la majorité des animaux complexes. Pourtant, une étude menée par une équipe de chercheurs de l’Université de Vienne révèle que les anémones utilisent un mécanisme moléculaire jusque-là associé aux bilatériens pour structurer leur corps. Cette découverte pourrait réécrire une partie de l’histoire de l’évolution animale.
Un mécanisme ancien partagé par des mondes éloignés
Le mécanisme en question est la « navette BMP médiée par la Chordine ». Derrière ce nom un peu technique se cache un processus clé du développement embryonnaire chez les bilatériens, c’est-à-dire les animaux qui présentent une symétrie gauche-droite, comme les humains, les grenouilles, ou les insectes. Ce système utilise des molécules appelées BMP (Bone Morphogenetic Proteins) qui agissent comme des messagers indiquant aux cellules leur position dans l’embryon et le type de tissu qu’elles doivent devenir.
Concrètement, l’inhibition locale des BMP par une autre molécule, la Chordine, crée un gradient de concentration dans l’organisme en développement. Selon la quantité de BMP présente, les cellules savent si elles doivent former le système nerveux central, les reins ou encore la peau ventrale. Ce processus établit ainsi un axe dorsal-ventral qui est fondamental pour organiser la structure corporelle des bilatériens.
Or, les chercheurs ont découvert que les anémones de mer, malgré leur organisation très différente, utilisent également ce même mécanisme de navette BMP médiée par la Chordine. Autrement dit, ce processus n’est pas une innovation propre aux bilatériens, mais un mécanisme évolutif beaucoup plus ancien, qui aurait existé bien avant la divergence entre cnidaires et bilatériens.
Une origine évolutive remontant à 600 millions d’années
La divergence entre cnidaires et bilatériens est l’un des événements majeurs dans l’histoire évolutive des animaux. Ces deux groupes ont des architectures corporelles radicalement différentes et sont séparés par des centaines de millions d’années d’évolution, estimées entre 600 et 700 millions d’années. La présence du même mécanisme moléculaire dans ces deux lignées suggère donc qu’il était déjà présent chez leur dernier ancêtre commun, un organisme préhistorique très ancien.
Cette hypothèse soulève plusieurs questions passionnantes. Premièrement, cela implique que les fondations moléculaires pour organiser un axe corporel complexe existaient bien avant l’apparition des bilatériens, ce qui réévalue notre compréhension de la complexité des premiers animaux. Deuxièmement, cela remet en question l’idée que les structures bilatérales se sont formées de manière totalement indépendante dans chaque groupe, laissant ouverte la possibilité que l’ancêtre commun des cnidaires et des bilatériens ait lui-même possédé une forme de symétrie bilatérale primitive.
Une complexité ancienne bien cachée
Ce que cette étude met en lumière, c’est que la simplicité apparente des anémones de mer masque en réalité une organisation biologique étonnamment sophistiquée. Sans cerveau ni système nerveux central, ces animaux utilisent néanmoins un système moléculaire avancé pour organiser leur corps dès le stade embryonnaire. Cette complexité ancestrale montre que certains outils évolutifs sont si fondamentaux qu’ils ont été conservés, voire partagés, entre des branches évolutives très éloignées.
David Mörsdorf, auteur principal de l’étude, souligne que ce mécanisme n’est pas universel même parmi les bilatériens. Par exemple, il est présent chez les grenouilles mais absent chez les poissons, ce qui suggère qu’il a pu apparaître et disparaître plusieurs fois au cours de l’évolution. Cette plasticité et cette longévité font de la navette BMP médiée par la Chordine un excellent candidat pour un mécanisme évolutif ancestral clé dans la structuration du corps animal.
Vers une nouvelle compréhension de l’évolution corporelle
Cette découverte est plus qu’une simple curiosité scientifique. Elle invite à repenser l’évolution du développement corporel chez les animaux, en intégrant des mécanismes très anciens partagés entre des groupes qui semblaient jusqu’ici très éloignés. En étudiant des organismes comme les anémones de mer, les scientifiques peuvent remonter aux origines profondes des processus biologiques qui ont permis l’émergence de formes corporelles complexes, y compris la nôtre.
Ainsi, ce sont peut-être au fond des océans, chez ces créatures sans cerveau, que se trouve le véritable plan du corps humain, écrit il y a des centaines de millions d’années.
L'anus pourrait avoir évolué à partir d'un trou utilisé à l'origine pour libérer le sperme
Extrait de l'article:
The anus is a wildly successful innovation, but how did it evolve? A genetic analysis suggests it probably began as an opening used to release sperm that later fused with the gut – an example of evolution repurposing structures.
“Once a hole is there, you can use it for other things,” says Andreas Hejnol at the University of Bergen in Norway.
It is thought that early animals evolved the mouth and gut before the anus, as some simple creatures such as jellyfish still have this body plan. They have to expel the remains of their last meal out of their mouth before they can eat again, says Hejnol.
One idea for how early animals evolved an anus is that their mouths split in two. However, in 2008, Hejnol showed that the key genes controlling the development of the mouth region are quite different to those for the hindgut, suggesting an independent origin for the anus – and now he thinks he has tracked it down.
Hejnol and his colleagues have been studying animals such as Xenoturbella bocki, a worm-like organism found on the seafloor with a mouth and gut, but no anus, which may be a living representative of an ancient group that was intermediate between the ancestors of jellyfish and the first animals with an anus.
Now, they have discovered that X. bocki has a separate opening for releasing sperm called a male gonopore. There is no female opening, as eggs are instead released through the mouth. The team also found that several of the key genes controlling the development of the hindgut in animals with an anus also control the development of the gonopore in animals such as X. bocki, suggesting an evolutionary link.
“What happened is likely that the hole [gonopore] existed, and the digestive system was close by,” he says. “And then they just fused. They connected to each other, and they made a common opening.”
“The data are beautiful and very convincing,” says Max Telford at University College London. “I’ve worked on Xenoturbella for a long time, and the fact that we’ve never noticed it having a gonopore is extraordinary.”
Tracing the origins of the anus is more than just idle curiosity, because it is thought that once animals had a through gut running from mouth to anus, it laid down a body plan still in use today. “The existence of almost all animals we see around us might have something to do with the invention of a through gut,” says Telford.
However, he doesn’t think X. bocki is pointing us in the right direction. He thinks the group of animals to which it belongs once had an anus with a connected gonopore, then lost the anus. In other words, according to Telford, this group appeared only after the evolution of the anus rather than representing the stage immediately preceding it. Hejnol thinks his own interpretation is more likely, but for now there is no way to bring about an end to the debate.
L'oreille interne des Néandertaliens révèle des indices sur leur origine énigmatique
New research on the inner ear morphology of Neanderthals and their ancestors challenges the widely accepted theory that Neanderthals originated after an evolutionary event that implied the loss of part of their genetic diversity. The findings, based on fossil samples from Atapuerca (Spain) and Krapina (Croatia), as well as from various European and Western Asian sites have been published in Nature Communications.
Neanderthals emerged around 250.000 years ago from European populations—referred to as "pre-Neanderthals"—which inhabited the Eurasian continent between 500.000 and 250.000 years ago. It was long believed that no significant changes occurred throughout the evolution of Neanderthals, yet recent paleogenetic research based on DNA samples extracted from fossils revealed the existence of a drastic genetic diversity loss event between early Neanderthals (or ancient Neanderthals) and later ones (also referred to as "classic" Neanderthals). Technically known as a "bottleneck", this genetic loss is frequently the consequence of a reduction in the number of individuals of a population. Paleogenetic data indicate that the decline in genetic variation took place approximately 110,000 years ago.
The presence of an earlier bottleneck event related to the origin of the Neanderthal lineage was also a widespread assumption among the scientific community. As such, all hypotheses formulated thus far were based on the idea that the earliest Neanderthals exhibited lower genetic diversity than their pre-Neanderthal ancestors, as consequence of a bottleneck event. However, the existence of a bottleneck at the origin of the Neanderthals has not been confirmed yet through paleogenetic data, mainly due to the lack of genetic sequences old enough to record the event and needed for ancient DNA studies.
In a study led by Alessandro Urciuoli (Institut Català de Paleontologia Miquel Crusafont, Universitat Autònoma de Barcelona) and Mercedes Conde-Valverde (Cátedra de Otoacústica Evolutiva de HM Hospitales y la Universidad de Alcalá), researchers measured the morphological diversity in the structure of the inner ear responsible for our sense of balance: the semicircular canals. It is widely accepted that results obtained from studying the morphological diversity of the semicircular canals are comparable to those obtained through DNA comparisons.
The study focused on two exceptional collections of fossil humans: one from the Sima de los Huesos site of Atapuerca (Burgos, Spain), dated to 430,000 years old, which constitutes the largest sample of pre-Neanderthals available in the fossil record; and another from the Croatian site of Krapina, this representing the most complete collection of early Neanderthals and dated to approximately 130.000-120.000 years ago. The researchers calculated the amount of morphological diversity (i.e., disparity) of the semicircular canals of both samples, comparing them with each other and with a sample of classic Neanderthals of different ages and geographical origins.
The study's findings reveal that the morphological diversity of the semicircular canals of classic Neanderthals is clearly lower than that of pre-Neanderthals and early Neanderthals, which aligns with previous paleogenetic results. Mercedes Conde-Valverde, co-author of the study, emphasized the importance of the analyzed sample: “By including fossils from a wide geographical and temporal range, we were able to capture a comprehensive picture of Neanderthal evolution. The reduction in diversity observed between the Krapina sample and classic Neanderthals is especially striking and clear, providing strong evidence of a bottleneck event.”
On the other hand, the results challenge the previously accepted idea that the origin of Neanderthals was associated with a significant loss of genetic diversity, prompting the need to propose new explanations for their origin. “We were surprised to find that the pre-Neanderthals from the Sima de los Huesos exhibited a level of morphological diversity similar to that of the early Neanderthals from Krapina,” commented Alessandro Urciuoli, lead author of the study. “This challenges the common assumption of a bottleneck event at the origin of the Neanderthal lineage,” the researcher stated.
Une supernova a-t-elle eu un effet sur l'évolution?
Radiation from an exploding star may have had a profound effect on the evolution of life on Earth, a new study suggests.
About 2.5 million years ago, the viruses infecting fish in Africa"s Lake Tanganyika underwent a mysterious and rapid explosion in diversity. Yet the exact cause of this change has remained a mystery.
Now, a new study has found that the upswing in the types of viruses found in the lake happened at the same time that our planet was being pummeled by cosmic rays from an ancient supernova — suggesting a possible link between the two events. The researchers published their findings Jan. 17 in The Astrophysical Journal Letters.
"It’s really cool to find ways in which these super distant things could impact our lives or the planet"s habitability," lead author Caitlyn Nojiri, an astrophysicist at the University of California, Santa Cruz, said in a statement. "We saw from other papers that radiation can damage DNA. That could be an accelerant for evolutionary changes or mutations in cells."
Lake Tanganyika, in East Africa"s Great Rift Valley, is one of the largest freshwater lakes on the planet; it spans about 12,700 square miles and divides four nations — Burundi, the Democratic Republic of the Congo (DRC), Tanzania and Zambia. The lake is home to more than 2,000 species, more than half of which aren’t found elsewhere. This means that, according to the World Conservation Union, "no place on earth holds such a variety of life."
One factor that may have driven this diversification is radiation, the study authors propose. Scientists already know that energetic particles in space, known as cosmic rays, can damage the cells of astronauts to cause accelerated aging and that bombardments from these particles could be responsible for the structural preference of biological molecules known as chirality. Yet just how much of a role these space rays played in the history of evolution is relatively unexplored.
To investigate this question, the researchers behind the new study dug up and examined core samples retrieved from the seafloor. They found that it was rich in an isotope of iron called iron-60, which is commonly produced by stellar explosions. By radioactively dating this isotope, they found that the iron-60 within their sample split into two separate ages: one that formed 6.5 million years ago and another that was 2.5 million years old.
To trace the origins of this isotope, the researchers simulated the sun’s movement through the Milky Way. They discovered that roughly 6.5 million years ago, our solar system and star passed through the Local Bubble — a lower-density region of the Milky Way’s Orion Arm that is littered with debris from exploded stars.
The analysis then revealed that the later spike likely came from a supernova, either from a group of young stars in the Scorpius-Centaurus group 460 light-years away, or the Tucana-Horologium group 230 light-years away. By conducting a simulation of a near-Earth stellar explosion, the scientists found that such an event would have rained cosmic rays upon Earth for 100,000 years after the initial blast, creating a pattern matching that of the spike found in the sediment.
If their assumptions are correct and this event actually happened, it would have been powerful enough to penetrate Earth’s atmosphere and snap DNA strands in half — explaining the coinciding explosion of diversity in viruses discovered in Lake Tanganyika.
Although the scientists cautioned that this connection is far from certain, it does raise the possibility that powerful cosmic events may have sculpted life on our planet more significantly than scientists first thought.
"We can't say that they are connected, but they have a similar timeframe," Nojiri said. "We thought it was interesting that there was an increased diversification in the viruses."
Comment créer un mammifère en neuf étapes évolutives (Smithsonian)
Mammals are familiar beasts. From a squirrel on a power line to a blue whale swimming through the sea, we share the world with more than 6,000 mammal species of all shapes and sizes. While we can easily distinguish a creature like a jaguar from a reptile or a bird in the modern world, however, mammals as we know them are the result of hundreds of millions of years of evolutionary changes. In fact, many of the key features that make us mammals evolved even before the dinosaurs.
Paleontologists have known for decades that mammals emerged from a broader, diverse group of creatures called synapsids. The very first synapsids of about 306 million years ago were small and lizard-like, but distinguishable by a single opening in their skull behind their eye socket. (We have a modified version of this hole, the space between your cheekbone and your cranium where a jaw-closing muscle runs through.) Nevertheless, a big evolutionary gap exists between a very early, lizard-like synapsid and modern mammals like ourselves. The following list will take you through nine of the essential evolutionary shifts that allowed mammals to thrive from the Age of Dinosaurs to this moment.
A toolkit of teeth
Most mammals have a dental tool kit of differently shaped teeth. In our own mouths, for example, we have incisors to nip with, canine teeth to puncture, and premolars and molars to crush and mash grub. The diversity lets mammals handle a great variety of food and make the most of our meals, whether it’s a wolf nipping the last muscle of an elk leg or an elephant chewing grass.
Paleontologists can see the beginnings of this differentiation in synapsids more than 295 million years old. Despite its lizard-like appearance, the sail-backed Dimetrodon was a synapsid and more closely related to us than any dinosaur or other ancient reptile. Such pre-mammal synapsids are often called “proto-mammals” as their anatomy set the stage for what mammals would eventually become. Dimetrodon, in particular, illustrates an early dental shift that mammals would later take to extremes. The ancient carnivore’s name means “two measures of teeth,” referring to the stark difference between the large, piercing teeth in the canine tooth position and smaller teeth behind it along the jaw. The difference is the beginning of what anatomists call heterodonty, or having differently shaped teeth in different jaw positions. The condition differs from most reptiles, which are homodont and have teeth about the same size and shape along their jaws. As early synapsids went about feeding on the plants and animals of their world, what started as basic, conical teeth were modified into different feeding specialties. Mammal teeth eventually became so diverse in shape and so distinctive that paleontologists often tell the difference between one species and another based on their dental details.
Long lost ribs
The mammalian backstory isn’t just one of gaining new features. Some ancient traits were lost and had a major influence on mammal evolution. One of the significant losses among mammal ancestors was gastralia, or belly ribs.
Early synapsids like Ophiacodon had thin ribs running along their bellies between their shoulders and hips. Synapsids of the time sprawled with their legs out to the side, like lizards, and so the belly ribs offered some extra protection from the rough ground. As synapsids continued to evolve during the Permian Period, however, they lost their belly ribs. Creatures like our cynodont ancestors, as well as the saber-toothed gorgonopsians, didn’t have belly ribs. Instead, organs like the heart and lungs would be enclosed in the rib cage, and lower organs, such as the stomach and intestines, would be held in by the body cavity and surrounding muscle. Even though the change left proto-mammals more vulnerable to injury across their abdomens, the shift afforded more flexibility in more upright postures with better up-and-down flexibility.
A new roof in the mouth
Ever since fishy creatures crawled out of swamps to drag themselves across the land, breathing while eating has been a problem. Among those early creatures, no divider was present between the nose and throat within the mouth. One big cavity led toward the very closely arranged larynx and pharynx, used for breathing and swallowing. If early tetrapods had their mouths full, breathing at the same time would be a challenge.
Synapsids evolved an anatomical solution to this problem, and they did so more than once. Several synapsid groups evolved a secondary palate during the Permian, or a shelf of bone that separates the nose from the mouth and throat. If you stick your tongue to the roof of your mouth, that’s the secondary palate. The separation allowed predatory synapsids, in particular, the ability to catch prey and feed while still breathing through their noses, letting them hunt and eat more efficiently than their predecessors. Among the groups with a secondary palate were the weasel-like cynodonts, such as the Triassic species Thrinaxodon, who passed the secondary palate on to their mammal descendants.
An earful of jaw
One of the key traits that makes mammals what they are isn’t something you can easily see on the outside, but tucked away inside the ear. The earliest synapsids, much like reptiles, had lower jaws that were made up of several different bones. Behind the tooth-bearing dentary were several other bones notched together like puzzle pieces leading to the jaw joint. Over time, however, synapsid jaws shifted. The dentary expanded to become the entirety of the lower jaw, a single bone that afforded synapsids stronger bites. Paleontologists see the shift in some early mammals such as the tiny, shrew-like Morganucodon. At the same time, the jaw bones closest to the back of the jaw became smaller and specialized to transmit vibrations to the ear, improving synapsid hearing. The incus, malleus and stapes of our inner ear are the remnants of these ancient jaw bones. By the early part of the Jurassic, about 191 million years ago, mammals had very sensitive ears that helped them navigate a dinosaur-filled world.
Fur and whiskers
Fossils preserving the body coverings of proto-mammals are rare. The few examples that paleontologists have found so far indicate that early synapids had scaly, lizard-like skin, which eventually shifted to smoother, softer skin through the Permian. The question is when synapsids evolved fur.
No matter the age, most synapsid and mammal fossils are not found with any indication of how furry they might have been. But there are a few clues about when fur and whiskers began to be important to synapsids. Whiskers are a modified form of hair, and the sensitive hairs send a great deal of information to the brain. By looking at the proportions of proto-mammal and early mammal brains, paleontologists found that mammal predecessors had fur and whiskers by about 240 million years ago. The timing coincides with when reptiles were proliferating, perhaps indicating that an insulating fur coat and whiskers to help navigate the dark allowed mammals to become more nocturnal and thrive at small size as the Mesozoic got underway.
Eye bones disappeared
Mammals don’t have bones in their eyes, but some of our ancestors and relatives did. Much like many reptiles and birds today, early synapsids had a circle of thin bones inside the eye called the scleral ring. Exactly what these bones do is still mysterious. Anatomists hypothesize that the bones are attachment sites for muscles that help animals change different viewing distances, or perhaps help support the eye during changes in pressure like diving deep or flying high. When cynodonts, a diverse group of weasel-like synapsids gave rise to the earliest mammals during the Triassic, however, the scleral rings were lost. Paleontologists hypothesize that the evolutionary miniaturization of early mammals might have something to do with the shift, as the supportive roles of the scleral ring were not needed at smaller sizes. Whatever happens, mammals don’t have to worry about potentially breaking an eye bone.
Walking postures shifted
Dimetrodon and other early proto-mammals had their bellies close to the ground. Such synapsids moved almost like monitor lizards or crocodiles, their bodies flexing from side to side as they walked. While such early synapsids would have been capable of bursts of speed, they weren’t especially quick and lacked the endurance seen in many mammals today. During the Permian, however, some synapsid groups began evolving more upright body postures. Their limbs took on more column-like arrangements, lifting the body higher off the ground and shifting motion to up-and-down movements of the spine rather than side-to-side. Standing taller, and losing their gastralia, allowed proto-mammals and mammal ancestors to move faster and more efficiently, and better forage for food and escape potential predators. Cynodonts, especially, evolved more and more upright body postures during the Triassic, setting up the way mammals move today.
Milk fueled mammal growth
Mammals aren’t the only creatures to make milk, but it’s as much a defining feature for us as our peculiar inner ear bones. Even the egg-laying duck-billed platypus makes milk, exuding the protein-rich substance from glands on its belly. The questions facing paleontologists are how and when milk evolved among mammals. Some experts place the origin around the rise of synapsids. Perhaps, as the lizard-like proto-mammals became accustomed to life on land, they oozed a protein-rich substance from their bellies that kept their eggs moist on dry land. Over time, the fluid changed and gained new uses, nourishing young that hatched out of eggs or were born live to help them grow faster. More fossil evidence will be needed to investigate and test these ideas, but clues from Jurassic mammals indicate that they were both making milk and weaning their fast-growing young.
Oxygène et évolution
Une étude, parue dans les Annales de l'Académie nationale américaine des sciences (PNAS) datées du 22 décembre, avance des observations intéressantes au sujet de l'évolution. Extraits:
«Nous avons été surpris d'observer que près de la totalité de l'accroissement en taille des organismes s'est produit lors de deux intervalles de temps distincts», relève Michal Kowalewski, professeur de géoscience à Virginia Tech et un des co-auteur de cette étude. «De plus ces intervalles ont suivi deux événements majeurs d'oxygénation» de l'atmosphère terrestre, ajoute ce scientifique.
«Le fait vraiment intéressant c'est que chacun de ces intervalles correspond à des moments de l'histoire de la vie terrestre marqués par une évolution dans la complexité biologique», relève Jennifer Stempien, chercheur à Virginia Tech et un des co-auteurs de ces travaux. «Le premier étant l'émergence de la cellule eukaroytique et la seconde le développement de la vie multicellulaire», précise-t-elle.
Les cellules eukaryotiques ont succédé aux cellules dites prokaryotiques, qui étaient les toutes premières formes de vie. Il s'agit de la plus simple de ces deux formes de vie à base de carbone et de la première à évoluer. Ces cellules se reproduisent par division. Les cellules eukaryotiques sont beaucoup plus grandes et avancées avec à l'intérieur des matériaux génétiques (ADN) contenus dans un noyau. Elles ont besoin d'oxygène pour survivre. Enfin, elles se reproduisent sexuellement et évoluent dans ce processus pour s'adapter à leur environnement.
C'est ainsi que durant les premiers 1,5 milliard d'années d'histoire documentée de la vie sur la Terre avec des fossiles - entre 3,5 et 2 milliards d'années - on trouvait seulement des formes de bactéries simples extrêmement limitées dans leur capacité à grandir faisant que la taille des organismes n'a pas changé jusqu'à l'émergence de formes plus complexes de vie il y a environ deux milliards d'années, expliquent ces chercheurs. Toutefois avant cela, un autre phénomène clé s'est produit. Il y a plus de trois milliards d'années, des bactéries primitives ont «inventé» la photosynthèse qui leur a permis d'utiliser la lumière du soleil et le dioxyde de carbone (CO2) pour se nourrir. Et comme les végétaux aujourd'hui ces bactéries émettaient dans ce processus de photosynthèse de l'oxygène dans les océans et ensuite dans l'atmosphère.
En quelque 200 millions d'années, les organismes sont passés d'une taille microscopique à la taille d'une pièce de monnaie.
(...) La vie comme simple cellule a stagné un milliard d'années de plus jusqu'à il y a 540 millions d'années, juste avant la transition entre l'ère Précambrien et le Cambrien marqué par une nouvelle augmentation de l'oxygène dans l'atmosphère qui a atteint 10% de sa concentration actuelle.
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