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The modern day horse has been an important aspect and resource for the human race for thousands of years. From a form of transportation, a tool for work, an extension of ourselves, to a beloved pet. The rise of civilization could not have become what it is today if it was not for the horse. But the horse has not always been the creature that stands tall and elegant as it does today. It also was not as widespread throughout the world either.
So how has the horse become what it is today? How did the ancestors of the modern horse evolve and what caused them to do so? Are there specific reasons why horses were absent in certain areas of the world? How could we better understand the evolution of the horse, since the evolution is still poorly understood? The fossil record has many gaps within it, and this causes the evolution of many organisms to not be understood very well.
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Filling these gaps or at least trying to will help understand how the horse has evolved, and possibly how other organism have as well. It will also help see how the horse could potentially change in generations to come.
Climate change has been a huge factor in the evolution of all life in Earth’s history. In a study done by Secord et al. (2012), the authors had looked at the size changes of early North American horses during the Paleocene-Eocene Thermal Maximum (PETM). This was a period of time where the Earth’s global temperature was about 8oC warmer than today’s temperature.
Their main focus was on Sifrhippus sandrae, one of modern day horses’ ancestors. Sifrhippus was not found in North America and Europe until the PETM. (Secord et al. 2012) This indicates that it had migrated into these continents, which made it an ideal mammal to use in this study.
The authors examined Sifrhippus teeth to determine what they were eating and how that had affected them. (Secord et al. 2012) From Sifrhippus’s first appearance in North America, it was about 5.6 kg in body size. (Secord et al. 2012) Over time, the body mass of S. sandrae decreased about 30%. The authors found that the average body then was about 3.9 kg. later on, the body size increase to about 7.0 kg. Secord et al. (2012) then test this data to see if it correlated with the changes in temperature. They tested this by comparing it to enamel of Coryphodon, a large water dwelling mammal. By studying the enamel of organisms, scientist can determine the air temperature in mid- or high latitudes. The enamel of Coryphodon compared the enamel of Sifrhippus demonstrated that they were correlated negatively, which meant that Sifrhippus was also correlated negatively by the temperature. As the temperature increased, the body size of Sifrhippus decreased. (Secord et al. 2012)
As stated in the prior paragraph, one of the changes that the horse’s ancestors went through is their teeth. This is due to the climate also changing the vegetation available for the equine ancestors to eat. In a separate study done by Mihlbachler et al. (2011), the authors studied the teeth of some of the earliest horses from about 55 million years ago (Ma) that were in North America. Modern day horses evolved to have high-crown molars, which is thought to be due to adaptation. (Mihlbachler et al. 2011) This is thought to have happened because the horses were constantly wearing down their teeth from the vegetation that they were consuming and the environment were changing constantly. The earliest horses in North America had very short-crowned molars that were very poorly developed.
Through time, horses developed shearing properties on their molars, which indicates that they started to change their diets from grasses to browsing. The confounding thing was that the subfamily of modern day horses appeared around 18 Ma, but that grass habitats had been present millions of years before their appearance. The authors began to compare the molars of main different species and how they began to wear out. What they ended up figuring out was that even though the climate was a huge factory to the molars changing, there was not a definite change that happened suddenly. What had happened was that as the climate changed, horses just dealt with what was going on. The variability of the selection was rather large. The selection of high-crowned molars did not occur until there was really heavy pressure to have them, since there were times that the environment was changing a lot. (Mihlbachler et al. 2011)
Size of the horse also had changed drastically during the PETM. According to Secord et al. (2012), the size of a mammal changes according to the temperature. Their study did agree with Bergmann’s law, which “is usually attributed to thermoregulation and the optimization of body size and/or the availability of food resources related to primary productivity.” (Secord et al. 2012) Basically, as temperature increases mammal body size decreases, and as temperature increases mammal body increases. This rule usually follows 65-75% of mammals, so it does not always apply to all mammals. The early North American horse had arrived to when temperatures were alright for its size, but as the temperatures for PETM increased, their body size decreased. This was mostly due to the changes in vegetation that were also occurring, which caused the horse to decrease in order to adapt, along with its changing molars.
The modern day horse was extinct in the Americas for thousands of years until their reintroduction during the arrival of Europeans to the Americas. These modern day horses flourished, and even at times today are called an invasive species due to their destruction of the country side where they roam today. But what caused them to disappear? In the study by Mihlbachler et al. 2011, the authors had indicated that since the environment was always changing in North America, the horse was not able to go ahead and evolve fast enough. There was an extreme pressure put on the horse due to vegetation change, which cause isolated populations to die out. Those that survived most likely traveled back to areas in the Old World which were much more suitable for them to survive.
Horses are a prime example for the process of evolution. The horse had gone through many different changes, both big and small throughout time. But the way that they had evolved is very poorly understood. It is important that we figure out a way to better understand how horses have evolved and where they fit into history, because then we may be able to better understand the evolution of other mammals that live today. For instance, we can see many differences just between breeds of horses. In a study done by Orlando et al. (2013), the authors analyzed the genomes of Late Pleistocene horse, modern five domestic horse breeds, a donkey, and a Przewalski’s horse. Through their data, they were able to identify that modern day horse rose sooner than previously thought, and that this Late Pleistocene horse was indeed the ancestor to all modern horse breeds, donkeys, and zebras as well. Their data also indicated that the number of horses fluctuated dramatically throughout time, specifically due to the severe changes in the climate.
The history of horses is important to understand how they have changed. It is also important to understand how they might change in generations to come. Horses have a rich amount of history just waiting for us to see, gather, analyze and process. By seeing their evolutionary changes, we can determine where they began, where they were losing, and how they were flourishing. Understanding all these things could help see where the horse has been, and how mammals around the horse could have also evolved. It helps us see how the environment has changed, and what affect it had on species. Continuing the study of the horse could mean the beginning of a new understanding of evolution.
The Evolution of the Modern Day Horse. (2021, Dec 11). Retrieved from https://studymoose.com/the-evolution-of-the-modern-day-horse-essay
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