?What types of dinosaurs did our human ancestors realize
A new study suggests that ancestors of humans — specifically, the ancient lineage of placental mammals, which includes primates, dogs or bats — and dinosaurs may have coexisted for a brief period before the famous fall of the asteroid.
It's been a long-standing debate among scientists: when did the key characteristics that define placentals ( Placentalia ) emerge? This very diverse intermediate class of mammals, of which humans are a part, is defined by the delivery of juveniles (as its name suggests) thanks to the presence of a placenta - in contrast to marsupials and their larvae (kangaroos , wombats, koalas, etc.) as well as monotremes and their eggs (echidnas, platypus ).
But did it develop before or after the mass extinction of the dinosaurs ? For the moment, no definitive fossil of this infra-class detected has been dated before this capital event of the Cretaceous-Paleogene (K-Pg extinction), 66 million years ago .
Thanks to a new statistical approach, researchers from the universities of Bristol (England) and Friborg (Switzerland) seem to have succeeded in showing that the first forms of placental mammals must have emerged during the Cretaceous… thus mixing with the dinosaurs for a short time. period. Their findings are published in the journal Current Biology on June 27, 2023.
Probabilities to obtain dates
To achieve these results, the scientists "collected thousands of fossils of placental mammals" , they say in a press release . They then used an in-depth statistical model called a "Bayesian Brownian bridge" (BBB).
As a reminder, the "clades" are groups of living beings bringing together both a common ancestor and all his descendants - these are ultimately the branches of the phylogenetic tree, which traces the kinship links between species by based on DNA sequences or proteins.
By drawing on fossil evidence and modern diversity, the method the researchers use ultimately allows the age of origin (and, if applicable, extinction) of these clades to be estimated.
These probabilities make it possible to determine the models of evolution over periods where there is no tangible proof to put in their mouths, as is the case for placentals beyond a certain period. Indeed, only a small number of animals reach the status of fossil, so many specific conditions must be met for an organism to be preserved.
Placental mammals alongside dinosaurs
In this new study and by accumulating data related to 380 families of placental mammals, the scientists therefore obtained "the patterns of origin and extinction of the different groups" , explains Emily Carlisle, paleobiologist at the School of Life Sciences. Ground of the University of Bristol and lead author. "Based on this, we [could] estimate when placental mammals evolved."
Analyzes suggest that the Placentalia lineage goes back further than they previously imagined — "just after the Cretaceous-Paleogene (K-Pg) boundary or earlier, during) the Cretaceous period" , according to previous interpretations fossil records and "molecular clock" data (cf. A more accurate statistical model in the future ).
They make it possible to estimate that the origin of 21.3% of placental mammals, including the groups that gave birth to primates , dogs and cats (in particular) as well as Lagomorpha (rabbits, hares), could have gone back until the Cretaceous, that is to say at the time of the dinosaurs.
The "Bayesian Brownian bridge" model employed here also shows that it was only after the asteroid impact that more modern lineages of placentals began to emerge. It is thus possible that the conditions for their diversification were better (and less competitive) after the extinction of the dinosaurs – and of approximately 76% of all the species which populated the planet.
A more accurate statistical model in the future
Moreover, the simulations obtained corresponded well to data previously obtained thanks to the method of the "molecular clock" (or "clock of evolution"), which already suggested that placental mammals had particularly ancient roots.
This other static model of the "molecular clock", a sort of time machine, makes it possible (in a very simplified way) to "rewind" the genetic changes (mutations) that occur regularly over time.
It thus makes it possible to theoretically estimate the absolute ages of divergence between species. In the same way, paleo-geneticists use it to date evolutionary events that the fossil record does not allow to document, because weak or even non-existent.
But the authors of the study indicate that their recent technique is even more precise, "for determining the evolutionary paths of species" , than these famous molecular data or than the use of fossil records. And this, in particular when the latter are missing. They now hope that the model they have developed will be used for further studies.
Source: websites