Fossils where do they belong? Maybe we didn’t look hard enough.

In 1996, paleontologists made an amazing discovery in northwest Madagascar. They appeared among dinosaur bones and sand sediments A small jaw piece, 167 million years old with three teeth. It belonged to the Ambondro mahabo, a species more than 25 million years old than any mammal of its kind ever found.

And he wasn’t supposed to be there. At the time, what was known of the fossil record overwhelmingly pointed to the conclusion that the ancestors of modern mammals originated in the northern hemisphere.

said John Flynn, a paleontologist who led the dig and is now Frick Curator of Fossil Mammals at the American Museum of Natural History in New York.

It takes more than one fossil to overturn the entire theory of evolution. But the Review of existing fossil holdings A publication last year in the journal Alcheringa sought to turn decades of paleontological wisdom on its head. After an extensive study of the skulls, jaws, and teeth, a team of Australian paleontologists has presented their conclusion that modern mammals originated in the southern hemisphere.

Their findings sparked spirited debate, revealing a North-South divide. Defenders of the northern hemisphere hypothesis highlight weaknesses they see in the latest discoveries. In response, proponents of a Southern Hemisphere origin, such as Dr. Flynn, say it is time for paleontologists to grapple with the argument that their field’s understanding of natural history may have been tilted toward the half of the world where scientists have conducted the most excavations. .

“In the Southern Hemisphere, these are just places that paleontologists haven’t explored,” said Dr. Flynn. “There was a long-standing overall bias in the system toward a northern hemisphere perspective, in part because that’s where the scientists came from. And it leads you to interpret a lot of things in light of that bias.”

At the heart of the controversy are the early Neanderthals of modern placental and marsupial mammals. Known as tribosphenic mammals, these mammals, Tim Flannery, an independent Australian paleontologist and one of the authors of the recent review paper, said, “small shrew-like creatures that would have weighed as much as a mouse.”

Although advanced for its time, it was a very basic version of the mammal as we know it today. Dr. Flannery compared them to the Ford T model of “modern or placental mammals.”

Dr. Flannery and company cite the geographic arguments in favor of the idea that early mammals could have originated in the Southern Hemisphere. The larger the Earth’s mass, the greater the likelihood of major evolutionary activity. When mammals appeared, Gondwana included Africa, India, Australia, and South America and was much larger than Laurasia in the northern hemisphere.

“There was a lot going on there,” said Dr. Flannery, referring to the appearance of songbirds and birds of prey on Gondwana during the age of the dinosaurs. “We just added this extra evolution that we think mammals were evolving here as well.”

The early mammals of the Southern Hemisphere were unlike anything our planet had seen before.

“They had uniquely complex teeth that allowed the animal to pierce its food, crush its food, and chop its food, all with the same tooth with different sides to it,” said Dr. Flannery. This, he said, gave them a huge advantage over other living things. He added, “When they got to the Northern Hemisphere, they took off and became very diverse very quickly.”

The oldest triposphinic fossils, from South America, date to 180 million years ago, with a clear line of other triposphinic fossils found in the Southern Hemisphere, including the Ambondro Mahabo, up to 100 million years ago. “By this point, the teeth had become a kind of Swiss army knife, a fully functional toolkit that had become the teeth of mammals,” said Chris Hilgen, chief scientist at the Australian Museum in Sydney and another author of the latest revision paper.

It was also around this time—between 100 million and 125 million years ago—that the first tribosphenic mammals appeared in the northern hemisphere.

Dr. Flannery and his colleagues argue in the study that Tribosphenic mammals, having evolved in the south, migrated north, hopping between the two supercontinents.

According to Dr. Flannery, this explanation fits with the theory that a new species of mammal had been evolving in the Southern Hemisphere for millions of years before suddenly appearing in the Northern Hemisphere.

“There is nothing that is clearly ancestral to these animals in the Northern Hemisphere, but there are many more of them in the Southern Hemisphere,” he said.

Not everyone agrees. Zhe-Xi Luo of the University of Chicago is among the advocates of the current hypothesis that tribosphenic mammals originated in the northern hemisphere. He said the Southern Hemisphere origins hypothesis was “disadvantaged by the loss of an enormous amount of data”.

Heavy emphasis on fossil molars, or teeth, at the expense of other parts of mammalian anatomy, argue Dr. Flannery and his co-authors. They also fail to consider fossils from all branches of the mammalian evolutionary tree. Moreover, says Dr. Lu, Dr. Flannery and his colleagues neglected to perform a computational analysis of the existing data. Such a statistical study would require building an extensive database of known fossils and using algorithms to compare anatomical features. It could also enable paleontologists to reconstruct patterns of phylogeny, and thus evolution.

Dr Flannery, who questioned the reliability of these databases, said the decision not to conduct such an analysis was deliberate and transparent. He said such analyzes lead to double counting of some items, and the database itself may be unreliable.

In Dr. Lu’s own work, he suggests that tribosphenic mammals likely appeared in China, regardless of whatever was going on further south. He says the southern tribosphenic mammals either died out or became monotremes, a family of mammals that includes the platypus and echidna.

Dr Flannery and co-authors also addressed the links between monotremes and tryposphinic mammals last year’s paper. In that paper, they argued that monotremes belong to a separate branch of the mammalian evolutionary tree. “Monocers have absolutely nothing to do with other modern mammals,” he said. “They are an older lineage” – a conclusion strongly disputed by Dr. Lu.

Guillermo Rougier, a University of Louisville paleontologist and reviewer of the paper by Dr. Flannery and colleagues, offered a cautious endorsement of the Southern Hemisphere origin argument.

He said, “It’s like a hammock with a ton stone on each end, and then you put two grains of rice on one side.” “You end up with a conclusion backed by a ton of evidence plus two grains of rice, but at the other end you have another conclusion backed by a ton of evidence.”

Neither side expects this paper to be the last word in the process of reconstructing mammals’ past.

“Right now, it’s like finding a long-necked fossil and making conclusions that confuse giraffe with the Loch Ness monster, because we don’t have enough information,” said Dr. Rougier.

Dr. Flynn said, “People think everything has been discovered in paleontology. Nothing could be further from the truth.”