Ancient skull reveals origin of big bird brains

Researchers at the University of Cambridge have described an extraordinary fossil of an ancient bird skull. The 80-million-year-old specimen, of an early bird species called Navaornis, is so well preserved -  and comes from a period that is so poorly understood in the evolutionary history of these animals - that zoologists think it could revolutionise our understanding of how modern birds came by their surprisingly big brains. The findings have been published in the journal Nature, and I went to meet Guillermo Navalon Fernandez to find out more…

Guillermo - Birds, modern birds, have very large brains and very complex brains and we actually don't know where and when these evolved, which is one of the biggest mysteries in vertebrate devolution.

Chris - Birds are direct descendants of dinosaurs, aren't they? So does that mean dinosaurs potentially were brainy? They were certainly the ancestors of birds. So if birds are brainy, perhaps they were too.

Guillermo - This is what we thought for a really long time. And definitely dinosaurs had more complex brains than other groups of reptiles. However, we know that they are still very far away from the condition that we see in modern birds. So even the first birds that we have three dimensional brains from, we can see the form of this structure like Archaeopteryx, the earliest bird that lived around 150 million years ago, these early birds have these brains that are still very similar to dinosaurs.

Chris - So there's a gap there. We've got very early birds, very similar to dinosaurs, their descendants, modern birds, big brains quite different. And we want to know what sits in that gap over, what, about 60-100 million years?

Guillermo - Yeah, so we are talking about a gap between 70 and 60 million years because we have Archaeopteryx 150 million years. And we know that the modern birds appeared at the end of the Mesozoic era or the age of dinosaurs at around 80 million years ago. Now we know a lot about the birds that came evolutionarily in between these two endpoints in evolution, but the fossils that we have from these birds come from fossils that are completely flattened. So this has opaqued our ability to reconstruct the three dimensional morphology of what's happening within the skull, which also includes the shape of the brain.

Chris - What have you got then in your hands now that helps to address this problem?

Guillermo - We've been working since 2021 in this spectacular locality in the south of Brazil and we found these very unique birds preserved in three dimensions. So that allowed us to reconstruct or have a very good representation of how the skull of these animals that have the brain inside were like so we could reconstruct the morphology of the brain of a bird that is right in the middle in the evolutionary journey between Archaeopteryx and modern birds.

Chris - What's the timeline then? When do these fossils date from?

Guillermo - We currently believe that these fossils date from between 80 and 70 million years ago, we are talking about the end of the era at the age of dinosaurs.

Chris - How big would the bird in life have been? How big was its brain?

Guillermo - We are talking about a starling size animal and the brain was more or less 10 millimetres end to end, which in a starling it would have been bigger than that. So we are talking about a brain that is relatively smaller relative to what we are used in modern birds.

Chris - It really is then that stepping stone. It's bigger than Archaeopteryx but smaller than a modern bird brain. So it is arguing that brains were beginning to get bigger in these birds as they began to evolve towards the tail end of the reign of the dinosaurs.

Guillermo - Exactly. This is precisely what we believe at the moment. This new brain is showing us that the size of the brain is increasing in these groups, but also is telling us the sequence of evolutionary changes that led to the origination of the modern brain. And by that I mean is telling us the sequence of expansion of the different areas of the brain because it was not a constant sort of expansion of the total brain. Some parts of the brain were actually expanding to modern standards while other regions of the brain were not expanding at the same pace.

Chris - Which bits were getting bigger. And in biology when things change in that way, they usually change for a reason. So what was driving this change?

Guillermo - So we are still unsure about these specific drivers. Some of the characteristics of this brain that are quite modern is for instance the general architecture of the total brain. So, by that I mean that the brain is starting to fold, to flex upon itself, kind of like what happens in humans. However, we see that many of the other regions of the brain, for instance the cerebellum, is still very flat and very small. And we know that this area of the brain is very important in modern birds to coordinate very complex mechanics of flight. So we have no idea how Navaornis and the rest of the group that Navaornis belongs to were flying effectively without this neuromotor coordination, this very complex neuromotor coordination, because we know from many other aspects of the anatomy that they must have been flying very well. As any scientific finding solves some questions, we are also left with millions of new questions that we have absolutely no idea what they mean.