The scientific community has long believed that spiders and their close kin evolved on land, but a new analysis of an exquisitely preserved fossil from 500 million years ago suggests otherwise. Researchers have discovered that these arachnids actually originated in the ocean, challenging the widely held assumption that their diversification occurred only after their common ancestor had conquered the land.

The study, led by Nicholas Strausfeld at the University of Arizona, analyzed the brain and central nervous system of an extinct animal called Mollisonia symmetrica. This ancient creature outwardly resembled some other early chelicerates from the Cambrian period, with a broad rounded carapace in the front and a sturdy segmented trunk ending in a tail-like structure. However, what Strausfeld and his colleagues found was that the neural arrangements in Mollisonia’s fossilized brain were not organized like those in horseshoe crabs, as could be expected, but instead were organized the same way as they are in modern spiders and their relatives.

The unique organization of the mollisoniid brain, which is the reverse of the front-to-back arrangement found in present-day crustaceans, insects, and centipedes, and even horseshoe crabs, is a crucial evolutionary development. Studies of existing spider brains suggest that this back-to-front arrangement provides shortcuts from neuronal control centers to underlying circuits that coordinate a spider’s repertoire of movements. This arrangement likely confers stealth in hunting, rapidity in pursuit, and exquisite dexterity for the spinning of webs to entrap prey.

The discovery of Mollisonia symmetrica as an arachnid ancestor has significant implications for our understanding of evolution. According to Strausfeld, it is still vigorously debated where and when arachnids first appeared, and what kind of chelicerates were their ancestors. The findings suggest that a Mollisonia-like arachnid may have become adapted to terrestrial life, making early insects and millipedes its daily diet.

The first creatures to come onto land were probably millipede-like arthropods and some ancestral insect-like creatures, an evolutionary branch of crustaceans. It is possible that these early terrestrial animals contributed to the evolution of a critical defense mechanism: insect wings, hence flight and escape.

Despite their aerial mobility, insects are still caught in millions in exquisite silken webs spun by spiders. The study’s findings highlight the complexity and diversity of life on Earth and remind us that even the most seemingly well-understood creatures can hold surprises waiting to be discovered.

The evolution from dinosaurs to birds included significant anatomical modifications. One crucial change was the development of a tiny wrist bone called the pisiform that helped stabilize wings in flight. A new study suggests that this bone appeared in bird ancestors millions of years earlier than first thought.

Paleontologists at Yale and Stony Brook University led a research team that made the discovery after examining fossils from two species of bird-like dinosaurs found in the Gobi Desert in Mongolia. The findings were published in the journal Nature.

“We were fortunate to have two immaculately preserved theropod wrists for this,” said Alex Ruebenstahl, a student at Yale’s Graduate School of Arts and Sciences. Both Ruebenstahl and his colleague Bhart-Anjan Bhullar noticed that the wrist bones were small and had shifted during decay and preservation.

The evolution of theropod dinosaurs into birds included significant anatomical modifications, such as changes in the pelvis and its surrounding musculature – and a transformation of the dinosaur forelimbs. One key change was the replacement of the ulnare with the pisiform bone in birds. In living birds, the pisiform is an unusual wrist bone that forms within a muscle tendon.

“This integration is particularly important for stabilizing the wing during flight,” said Bhullar. The discovery suggests that the integrated pisiform evolved prior to modern avian flight, and was diminutive in these near-bird dinosaurs, which is consistent with their limited flight capabilities.

The research continues a rich Yale tradition of advancing understanding of bird evolution from dinosaurs. In the 1960s, Yale paleontologist John Ostrom identified another wrist bone found in both meat-eating dinosaurs and modern birds. In the 1980s, Yale paleontologist Jacques Gauthier definitively showed that this wrist bone linked dinosaurs to birds.

The study’s lead author, James Napoli, of Stony Brook University, collaborated with Ruebenstahl and Bhullar to re-identify wrist bones in other dinosaurs as pisiforms. Further analysis showed the unidentified bones were indeed pisiform bones. The researchers then expanded their efforts to study the development of some of the flight muscles associated with the pisiform.

The research was conducted by a team including co-authors Matteo Fabbri, Jimgmai O’Connor, and Mark Norell, and furthered understanding of bird evolution from dinosaurs.

[Image description: A sun-kissed landscape with towering trees, sparkling rivers, and vast plains stretching out to the horizon. In the foreground, a group of early humans are seen huddled around a fire, roasting meat on skewers while others are busy gathering fruits and berries from nearby bushes. Nearby, a Neanderthal individual is spotted, using a crude stone tool to scrape off flesh from a freshly hunted mammoth carcass. The two groups seem to be living in harmony, with some members of each group occasionally interacting and exchanging goods or stories.]

The Princeton study, led by Joshua Akey, has uncovered new layers of the shared history between early humans and Neanderthals. Using a genetic tool called IBDmix, the researchers mapped gene flow between the hominin groups over the past quarter-million years, revealing multiple waves of contact and interbreeding.

“We now know that for the vast majority of human history, we’ve had a history of contact between modern humans and Neanderthals,” said Akey. The results of their work were published in the journal Science, challenging previous genetic data that suggested modern humans evolved in Africa 250,000 years ago and then dispersed out of Africa 50,000 years ago.

The study found evidence of three main waves of contact: one about 200-250,000 years ago, another around 100-120,000 years ago, and the largest wave about 50-60,000 years ago. This contrasts sharply with previous research that suggested modern humans stayed put in Africa for 200,000 years before dispersing out.

The researchers also discovered that Neanderthals had a smaller population than previously thought, with estimates revised from around 3,400 individuals to roughly 2,400. This finding helps explain how Neanderthals disappeared from the fossil and genetic record around 30,000 years ago.

Akey’s team found strong evidence consistent with Fred Smith’s hypothesis that Neanderthals were largely absorbed into modern human communities rather than going extinct. “Our results provide strong genetic data consistent with Fred’s hypothesis, and I think that’s really interesting,” said Akey.

The study provides a new understanding of the complex relationship between early humans and Neanderthals, highlighting the importance of interbreeding and cultural exchange in shaping human evolution.