Extinct animals fascinate us. Huge extinct animals also amaze us. There are small animal fossils that are magical, precious, perfectly preserved, and fundamental to our understanding of the evolution of life on Earth. But when, at the age of six, he sees a Diplodocus skeleton for the first time in his life, part of his world collapses. Although you don’t quite understand how, you suddenly realize that such a gigantic mistake existed. The unimaginable becomes real, because if you reach out a little, you can touch their bones.
The gigantic extinct animals such as mammoths or large dinosaurs suddenly let our imagination fly into almost fantastic prehistoric worlds. The fascination for these primeval animals is universal and casts a spell over laypeople and experts alike. It is not surprising, then, that many paleontologists have tried to understand how some groups of animals became so large.
The thing has its crumbs, because there have not always been giant beasts. Consider ecosystems just after the late Cretaceous extinction period 66 million years ago, when an asteroid wiped out non-avian dinosaurs. The largest land animals to survive this event rarely weighed 10 kilos. Around 15 million years later, it is already teeming with mammals weighing several tons. How did it happen?
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We paleontologists have pondered this question for almost 200 years without coming to a clear conclusion. So a few years ago we set out to find an answer. For our study, the results of which are published today in the journal Science, we focused on brontothera, distant relatives of tapirs and rhinos that inhabited the planet in the Eocene 56 to 34 million years ago. The name Brontotherium means ‘beast of thunder’, and some of the best-known species boasted imposing flat, forked horns on their noses.
The first Brontotheres species weighed about 20 kilos and the last five tons (like a modern elephant). For this reason, it is very interesting to learn about the evolution of body size in mammals. Using mathematical models simulating evolutionary processes and the most accurate data available, we compare the different theories that have been developed over the past two centuries to explain the evolution of these titans. This was a journey through the history of evolutionary theory, the different perceptions we had about the nature that surrounds us and ultimately the origin of biodiversity.
Some late 19th-century naturalists, known as neo-Lamarckists, believed that animal lineages were destined to evolve in their ecology into increasingly specialized forms that were larger and generally more bizarre (e.g., the evolution of horns of all kinds and strange skull protrusions). This tendency was unresponsive to adaptations of the medium, ignoring Darwin and his already published adaptationist ideas. Rather, they believed that the evolutionary history of lineages was pre-programmed from the start, like some kind of assembly instructions. His ideas were influenced by Jean-Baptiste Lamarck, a French naturalist who believed that evolution followed an inevitable ladder toward complexity. Neo-Lamarckists, for example, held that the various lineages of mammals (brontotheres, horses, elephants, etc.) were destined to repeat a similar evolutionary path. Only then could they explain the very clear trends they kept seeing in the fossil record. The mechanisms proposed by Darwin (the success of the fittest in the eternal struggle for survival) were too chaotic to explain such linear evolutionary paths: from small and unspecialized to large and specialized.
Darwin could not explain gigantization
At the beginning of the 20th century, thanks in particular to the development of genetics, Darwinian ideas prevailed. However, the persistent fossils showed that many groups of animals did in fact appear in small forms and increased in size over time. In order to fit them into Darwinian postulates, the old ideas of neo-Lamarckism had to be recycled and explained in new terms: bigger must be more advantageous, and large individuals must be fitter, so that in the end natural selection operating in minute detail over millions of years , will end with clear trends towards larger and larger formats. Because it was a repetition of the ideas of Edward D. Cope, one of the most influential neo-Lamarckian paleontologists, this law of evolution has been dubbed Cope’s rule.
To believe this idea, one has to accept that it is indeed always more beneficial to be larger, in vastly different groups and over millions of years. Only in a very predictable world could one imagine that the effect of natural selection, acting at the level of the organism (this one is fit and will leave more offspring; this other one, however, does not persist with us) can have on evolutionary tendencies are extrapolated, which remain unchanged for tens of millions of years. However, the myriad of climatic and ecological conditions that occur over such long periods of time are rarely so stable and homogeneous.
Different sizes in Brontotheres species. Below: “Eotitanops borealis”, one of the first and smallest species of the group. In the background “Megacerops coloradensis”, one of the last giants to survive until the end of the Eocene 35 million years ago.ÓSCAR SANISIDRO MORANT (UNIVERSITY OF KANSAS)
In fact, the neo-Lamarckian idea of predictability in evolution (and thus the notion of extrapolation) was finally discarded in the 1970s, when a number of new theories were developed that helped to reconcile more Darwinian principles with the data of the fossil record. Natural selection is still the main engine of evolution, albeit with some changes. In the case of Cope’s rule (remember, a general trend toward larger sizes), this could be explained as follows: natural selection occurs in response to immediate conditions, here and now.
Therefore, the resizing of animal populations will occur in response to some very specific circumstances. When a new species emerges, it may be larger or smaller than its ancestor, depending on conditions. This step seems easy to accept, but we’ve just removed the ability to extrapolate. Being great no longer always means being fit, it depends on the surrounding circumstances. And if the descendant species can be larger or smaller, we will never see a clear trend towards size increase for many millions of years. If natural selection doesn’t provide the direction, how do we explain the trends we sometimes see in the fossil record, such as toward larger sizes?
In the 1970s, theories were developed that made Darwinian principles compatible with the fossil record. Natural selection remains the main driver of evolution, albeit with changes
To better understand this trade-off, let’s assume that instead of an evolutionary tree, we have a bonsai and we want it to only grow in one direction (towards larger sizes). We have two options. The first is to gradually force all the branches of the tree in that direction, for example using guides and wires. This option would reflect Cope’s rule proposed by Neo-Lamarckists, since all branches tend to go in the preferred direction (towards larger size). The new branches orientate themselves towards the desired direction rather than the branch that spawns them. That is, the descendants are always larger than the ancestors. However, the paleontologists of the 1970s proposed a second option: bonsai branch freely in all directions. New branches can appear further to the right or left than the branch from which they are derived. How do we make the bonsai grow sideways only? Well, one uses pruning shears that only cut the branches on one side, allowing the branches to spread only in the desired direction.
In the context of size evolution, these secateurs represent an extinction that primarily affects the smaller lineages and only reproduces the larger lineages. The tendency is not caused by gradual unilateral changes resulting from the natural selection of organisms, but by a process that selects and prunes whole branches. Branching and scissors are two different processes. Both bring change, but to different degrees, as the scissors are much more effective. This new perspective shows us a much less predictable world, since the scissors represent the extinction caused by unpredictable factors: environmental changes, catastrophic events, competition with new groups of animals, etc. From this perspective, it is difficult to predict the shape of the bonsai, since we do not know a priori where the scissors will cut. In summary, our theories of evolution have shifted from those that proposed a predetermined order of evolution (which reminds us in part of the idea of a divine plan as defended by natural theology) to more unpredictable, chaotic notions of where Coincidence It takes more and more control.
The Brontotheres and the Scissors of Evolution
Ok, back to the Brontothers. Which of these theories best fits the evolution of these Eocene titans? Our analyzes now published in Science rule out that Brontothere lineages always increase in size, as predicted by Cope’s rule. Instead, they suggest that the bonsai and secateurs model better fits the data. That is, the new species were not systematically larger than their ancestors. But once the new species became established, the smaller species faced greater threats of extinction. Because? Because herbivore ecological communities at the time were full of small and medium-sized species, the typical medium-sized ecological niches were more saturated and the smaller Brontotherium species had more competitors.
In other words, the pruning shears worked the branches of smaller brontotheres harder. As larger species emerged, they escaped this competition and survived longer, allowing them to give rise to other species. In this way, the larger Brontotheres species became progressively more abundant than the smaller ones, leading to the pattern we see in the fossil record.
Since secateurs are factors that are difficult to predict, this finding teaches us that the brontothera were not predestined to increase in size. It was chance and coincidence that projected its development to gigantic proportions. If we could rewind evolution to 66 million years ago and press play again, the Brontothers most likely wouldn’t repeat the same path. Our discovery leads us to a less predictable and therefore unrepeatable development.
Oscar Sanisidro Morant He is a paleontologist from the University of Alcalá and scientific illustrator and lead author of the cited study.
Juan Lopez Cantalapiedra He is a paleontologist and researcher at the University of Alcalá.
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