Researchers in the lab of Brown neuroscientist Alexander Fleischmann have compared the mouse olfactory cortex, which governs the sense of smell, with the cerebral cortices of salamanders, lizards and turtles–and discovered remarkable similarities right down to individual cell types.

Essentially, said Fleischmann: “There is a piece of lizard brain in the mammalian brain.”

Fleischmann, the Provost’s Professor of Brain Science and an affiliate of the Carney Institute for Brain Science, made this discovery with an international team of scientists at Columbia University, Harvard Medical School, the Inria Lyon Centre in France and the Weizmann Institute of Science in Israel. The results, published in Nature Neuroscience, offer a never-before-possible look into how the brains of mammals, including humans, have changed over hundreds of thousands of years.

Mammals, reptiles and amphibians share a common vertebrate ancestor, but their brains evolved differently–and at different times in prehistory–to help them occupy different ecological niches. The first amphibians branched from the tree of life 350 million years ago; the first reptiles did so 300 million years ago; the first mammals did so 200 million years ago.

Despite these millions of years of divergence, this fact has long intrigued scientists: while the mammalian neocortex, which processes information from the other senses, has six layers of tissue, the mammalian olfactory cortex has just three layers of tissue. So do the cortices of reptiles and amphibians. Why? 

Evolutionary biologists and brain scientists have long speculated that the mammalian olfactory cortex was, evolutionarily speaking, an older structure. The Fleischmann Lab and their collaborators have proved this theory through state-of-the-art molecular analyses and machine learning. 

Their work contains two key findings. 

First, there is little difference between the olfactory cortices of mice and the entire cortices of salamanders, turtles and lizards–right down to cell type. This was a surprise according to Sara Zeppilli, who conducted this research as a PhD student in the Fleischmann Lab and who now serves as a postdoctoral researcher at Harvard.

“It's remarkable that despite 200 million years of side-by-side coevolution within the same brain,” Zeppilli said, “neurons from the mammalian olfactory cortex are more similar to the neurons of non-mammals than to neurons of the mammalian neocortex right next door.”

The second surprise: the mouse olfactory neurons studied came from adult mice–but many of these neurons resemble cells found in young mice.

Fleischmann and Zeppilli had a hunch these immature-seeming cells were conserved over millennia for their ability to help creatures quickly adapt to new environments. Data they obtained from comparing olfactory neurons from adult lab mice with adult wild mice supported this theory. Some olfactory neurons from the wild mice appeared different, suggesting that the cells adapted in order to help the wild mice survive.

“One way to generate adaptability is by having immature, plastic neurons and then have them take on functions that support whatever unexpected needs are out there,” Fleischmann said. 

Fleischmann speculates this adaptability may come into play in humans when a mother experiences a sharpened sense of smell during pregnancy, for example. The heightened sensitivity could be an ancient protection mechanism kicking in, helping to steer the mother away from eating certain foods that could cause illness or infection and harm her baby. The Fleischmann Lab intends to follow up on this line of thinking as a next step.

The team’s work was possible through the use of single-cell multiome sequencing, a new method for quickly and comprehensively comparing and defining different cell types. “Multiome” refers to the technique’s ability to analyze several aspects of the biology of single cells, including which genes are accessible to those cells and whether the cells are turning genes on or off. 

Maria Tosches at Columbia University, a co-author on the paper, was one of the first scientists to use sequencing methods to compare all the genes of individual cells, many cells at a time, across species, publishing datasets on turtles and lizards, and, later, on salamanders.

Zeppilli generated datasets for the mouse neocortex and the mouse olfactory cortex, and then compared it to Tosches’s datasets. Zeppilli hopes the new dataset created at Carney will inspire and help other researchers to keep uncovering our evolutionary story. 

“With this paper, we’re putting out a resource for other researchers to take this new, deeply-characterized olfactory cortex dataset and continue to build on it.”

Portions of this research were funded by NIH NIDCD R01DC017437 and R01DC020478 awards. Zeppilli was the recipient of a 2021/2022 Carney Graduate Award in Brain Science, supported by the Robin Chemers Neustein Graduate Fellowship fund and the Macklin Bequest.