Where primates evolved

[Music] Welcome to Science Sessions, the podcast of the proceedings of the National Academy of Sciences, where we connect you with Academy members, researchers and policymakers. Join us as we explore the stories behind the science. I'm Paul Gabrielson. Picture a primate, a monkey, ape or lemur. You're likely picturing them among green trees in a tropical environment. But precisely where primates originated is still a matter of scientific debate. In a recent PNAS study, Jorge Avaria-Yotreo and Chris Venditti of the University of Reading in the United Kingdom and colleagues, explored the geographic and climatic origin of primates. The result suggests a dry, cold evolutionary origin rather than the warm, leafy tropics into which primates later dispersed. Jorge introduces to the prior prevailing theory that primates originated and evolved in warm tropical forests. This idea about the tropical forest origin for primates has been the main idea for more than four decades. There are many lines of evidence supporting this tropical forest origin for primates. The main line of evidence comes from the extent primates diversity. If we look at the morphological characteristics of most primates, we can see that they possess many characteristics that are highly adapted to the tropical forest. For example, all of them have the opposable plumb which allow them to grasp branches to move agocetries. They also have the binocular vision which allow them to catch their prey insects, fruits on the branches. And then, if we look for example at the average temperature where they live, we can see that most of the temperatures are also very high temperatures. So given all these observations, the most simple inference when we go to the past is to hypothesize or assume that the climates in the past was the same, a tropical forest with warm temperatures. There is another common observation that is associated to the tropical forest origin, which is that most of the primate fossil record is from about 55 million years ago and it is spread across North America, Europe, Asia. And at the same time, there was a huge increase in global temperature. It was a short period of time in which the temperature increased about 5 to 8 Celsius degrees the mean global temperature. So this coincidence also gives more support to the idea that primates originated in a warm environment like a tropical forest. Chris, why did you think this might not be the case? We actually know that both from fossil evidence and statistical reconstructions that the primates that would have been living maybe 50 or 60 million years ago didn't look like the primates we see today. So there would have been much more tree-shrue-like or squirrel-like and that kind of thing. And as we know, those kind of animals can live in lots of different environments like we see today. But also, where the fossils are found 50 or 60 million years ago, some parts of North America aren't likely to have had the tropical climate there at the time. Recently, evidence from paleoclimate models and other climate proxies like pollen data are pointing towards the fact that those environments weren't actually a tropical at all. They were more temperate. So those two lines of evidence together suggest that it might not be quite as simple as the classic textbook theory suggests. How did you reconstruct the climate conditions of primate evolution? Mainly, we used two types of methods. One is a phylogenetic method that allowed to travel to the past and reconstruct the location of ancestral species in a phyloyony. So a phyloyony is a representation of the evolutionary relationships of species. And it mainly contains those species that we cannot see, which are the common ancestors of all the group that we are interested on. And this method used mainly data on the extent distribution or locations of primates and also the locations of the fossils. So then, after getting these inferences about the ancestral locations, we complemented this output with paleoclimate simulations of temperature and precipitation. So we took this information of temperature and precipitation to reconstruct the type of climate. And then we extracted this type of climate from the locations that we reconstructed with this phylogenetic method. Why was it important to use standardized climate definitions in this work? It links back to the prevailing hypotheses. Normally, or historically, those hypotheses have been built in a kind of narrative form. So people say warm forests, tropical forests, without any strict definition of those. And what we've tried to do is bring that into a statistical framework to actually explicitly test these hypotheses. And to do that, it's really important to standardize that so that everyone can see what we mean by tropical environments. And there's a particular way that we do that, which is using a well-known scale of temperature, which is quite widely accepted. Researchers used to identify the type of climate based only on the temperature or some reconstruction on temperature. So for example, if they find some evidence of hot temperatures in the past where the fossil primates are located, they used to assume that was a tropical climate. By temperature by itself is not enough to define the type of climate because, for example, we can find tropical climate where the temperature is over 18 Celsius degrees over the year. We can find a dry climate where the temperature can reach degrees over 20 or even we can find coal climates where the temperature in summer is over 20 Celsius degrees, but in winter drops below zero. So that is why it's not enough to get information of the temperature. We also need the variation of temperature, the seasonality through the year, and also the pattern of rainfall to define in a more complete way the climate. The other interesting thing that we found is that the warm tropical hypotheses that people have proposed before were also based on global temperature mostly, but the methodology we develop and use here actually uses the local temperature or tries to estimate the local temperature where the species actually lived at the time. So that's another point of difference that is probably really important in this study compared to many others and we hope that that inspires others to use the similar methodology for other groups to see if they're a similar or different patterns for that matter. What did you results tell you about the location and climate of primate evolution? The general picture that we got after doing our analysis is that the early primates were living outside the tropics. Most of them were living in North America, some parts of Europe and Asia, and then we discovered that they started to disperse over evolutionary time scales towards tropical latitudes where they live today. And in this same process of a geographical dispersal, we discover also that primates were colonized in different type of climates. So they started in coal climates, which are more seasonal, and then they started to move to other type of climates that are more stable, like for example the tropical forest. Why is it important to understand the climate during the evolutionary history of primates? As an evolutionary biologist or a biologist in general, we know that the climate is the main driver for diversity. So if we want to understand how the world came to be like it is today, climates are really important driver for those changes. It can affect a lot of different things about an animal. You can make some predictions about its morphology, about its basal metabolic rate, its biochemistry. It drives a lot of that diversity both in speciation and in morphological diversity. So in an academic sense, it's really important to understand that. But also if we can understand the changes that species evolved through in the past, we can compare those to what species are encountering today. And that might enable us to understand when or where or for how long species are able to cope with the change that is happening at the moment. What are the caveats or limitations of this study? The fossil record is incomplete. So there are many bias about the information that we can get from the fossil record. We are also using statistical modeling, which by nature has uncertainty in both. But we run many types of sensitive analysis just to mitigate this potential bias from the fossil record or they are sensitive to the statistical models. And every time when we run new analysis to test this potential bias, we are getting the same output, the same result, the same type of climate, the same pattern. constructing a phylogenetic tree of species that are extent and extinct is very difficult thing and obviously can be error-prone but we tried to overcome that one by repeating or integrating our results over lots of different phylogenetic trees to demonstrate that perhaps that isn't a problem in our results. Thanks for tuning in to Science Sessions. You can subscribe to Science Sessions on iTunes, Spotify or wherever you get your podcasts. If you like this episode please consider leaving a review and helping us spread the word.