Horses, this elegant and robust animal, are not only loyal companions of human civilization, but also classic examples of biological evolution. From the forest beasts the size of foxes 55 million years ago to the tall horses galloping on the grasslands today, their evolutionary history is a perfect witness to natural selection. As the Earth's environment shifted from dense forests to open grasslands, the size, limbs, and teeth of horses underwent astonishing adaptive changes, ultimately shaping the single hoofed sprinter we know today.

In the early Eocene, 55 million years ago, a small animal called Hyractherium lived in the dense forests of North America and Europe. It is also known as the ancestor horse and is the oldest ancestor of modern equines. The body size of the first horse is only the size of modern foxes or medium-sized dogs, with a shoulder height of about 30 to 50 centimeters and a weight of only 5 to 10 kilograms. Its appearance is more like that of today's horseshoe rabbits or small deer, rather than the familiar steed. Its body structure adapted to the warm and humid forest environment at that time: its back was slightly curved, its limbs were slender but flexible, its front feet had four toes, its back feet had three toes, and its toe tips had soft pads instead of hard hooves. This structure allowed them to move flexibly in soft forests and swamps, avoiding getting stuck in damp soil. The head of the first horse is relatively small, with eyes located on both sides and a wide field of view, which helps to detect predators in the jungle; The teeth are low crowned and hilly, suitable for chewing tender leaves, fruits, and soft plants, indicating that it is a mild herbivorous animal that feeds on forest understory vegetation. Although the appearance of the primitive horse differed greatly from modern horses, its skeletal structure, especially the unique arrangement of vertebrae, limb joints, and teeth, had already demonstrated the basic characteristics of the horse family, laying the foundation for the later evolution of horses. With the change of Earth's climate, forests gradually decreased and grasslands began to expand. The descendants of horses had to adapt to the new environment and gradually evolved into more suitable running bodies, more efficient digestive systems, and harder teeth through millions of years of natural selection, ultimately forming the horses we see today.

About 30 million years ago during the Oligocene epoch, as the global climate gradually cooled and dried up, the dense forests of North America began to be replaced by open forests and grasslands, and the evolution of the horse family also ushered in a critical turning point - the emergence of the Mesohippus. As a descendant of the ancestor horse (Hyractherium), the size of the middle horse has significantly increased, with a shoulder height of about 60 centimeters, equivalent to the size of modern medium-sized dogs, and a weight of up to 25-50 kilograms, indicating a trend towards larger horses. Compared to its ancestors, the four limb structure of the Chinese horse has undergone significant changes: the three phalanges (second, third, and fourth toes) of the forelimbs are still preserved, but the middle toe (third toe) has become thicker and bears most of its weight, while the lateral toes (second and fourth toes) have noticeably degenerated and no longer touch the ground, serving only as auxiliary support structures. This adaptation makes the middle horse more efficient when running, able to better adapt to gradually open environments and avoid predators' pursuit. At the same time, the back of Zhongma has become flatter, the leg bones are longer and the joints are more flexible, the overall body shape is closer to modern horses, and the running ability has been greatly improved. In terms of head structure, the face of the middle horse is longer than that of the ancestor horse, with eye sockets moving backwards and significant changes in teeth: the crown of molars is raised, forming a simple wrinkled structure on the surface, which can more effectively grind down rougher plants. This indicates that their diet gradually shifted from the tender leaves and fruits of the ancestor horse to tougher shrub branches and early grasses. In addition, the brain capacity of China and Malaysia has also increased, indicating higher levels of intelligence and possibly more complex social behaviors. Fossil evidence suggests that horses have developed small group life, which helps them increase their vigilance against predators in open environments. As a key link in the evolutionary history of the horse family, the Chinese horse not only filled the transitional stage from multi toed forest animals to single toed grassland runners, but also marked the beginning of horse adaptation to radiation, paving the way for further evolution of the grassland ancient horse (Merychippus).

During the Miocene period about 20 million years ago, as the global climate became increasingly arid, the grassland ecosystems on the North American continent rapidly expanded, and the horse family experienced the most critical adaptive breakthrough in evolutionary history - the emergence of the grassland ancient horse (Merychippus). As a descendant of Mesohippus, the ancient grassland horse represents a decisive stage in the transition of horses from forest animals to grassland animals. Its shoulder height has reached about 1 meter, and its body size is similar to modern small horses, weighing about 100-150 kilograms, showing a clear trend of increasing body size. The most significant evolutionary breakthrough is reflected in its limb structure: although it still retains its three toe form, the middle toe (third toe) has fully borne the weight of the whole body, and a hard hoof like structure has developed at the end, while the toe bones on both sides (second and fourth toes) are extremely degraded, leaving only small residual bones. This specialized structure makes it a true "hoofed" animal, able to run at high speed and persistently on open grasslands to avoid predators such as saber toothed tigers. A more revolutionary change occurred in the dental system: for the first time, ancient horses on the grasslands evolved high crown teeth, with a crown height of up to 10 centimeters, and formed complex wrinkled enamel structures. This adaptive feature allowed them to effectively grind hard grasses rich in silica, marking a complete shift in horse feeding habits from tender leaves to rough herbivores. The skull structure has also undergone significant changes: the face has been significantly elongated to accommodate the growth needs of high crown teeth, and the position of the eye sockets has shifted backwards and increased, giving it a broader field of view to alert natural enemies in grassland environments. The brain capacity has increased by 30% compared to its ancestors, indicating a significant improvement in its social behavior and cognitive abilities. Fossil evidence shows that grassland horses have formed a stable population structure. As the most important transitional type in the evolutionary history of the horse family, the steppe horse not only laid the foundation for the basic body configuration of modern horses (Equus), but its multiple subspecies also evolved different adaptive characteristics. One of them eventually evolved into the single toed true horse (Pliohippus), becoming the common ancestor of all modern horses, zebras, and donkeys.

During the Pliocene period about 5 million years ago, with the further expansion of the global grassland environment, the horse family evolved a highly specialized true horse (Pliohippus), which is the direct ancestor of the modern horse genus (Equus) and the first species in the evolutionary history of the horse family to fully form a single hoof structure. The body size of a real horse is close to that of a modern horse, with a shoulder height of about 1.2-1.5 meters and a weight of 200-300 kilograms. Its most significant evolutionary breakthrough is the complete reform of its limb structure: the lateral toes have completely degenerated, leaving only a developed middle toe (third toe), and the end is wrapped in hard keratinized hooves. This single hoof structure makes it a true "odd hoofed animal", able to run long distances on open grasslands with extremely high efficiency, reaching speeds of over 60 kilometers per hour, perfectly adapting to the survival needs of large predators such as saber toothed tigers. The skull structure presents typical modern horse features: the face is further elongated, the eye sockets are moved backwards and enlarged, providing a broader field of view; The dental system continues to specialize, with a crown height of up to 15 centimeters for high crown teeth and a more complex enamel fold structure, which can effectively handle increasingly rough grasses. The brain capacity of true horses is 20% larger than that of grassland ancient horses (Meryhippus), exhibiting stronger sociality and more complex behavioral patterns. Fossil evidence suggests that they have formed a stable group structure, and there may be a preliminary hierarchical system among group members. As the last crucial link in the evolution of the horse family, the true horse spread from North America to Eurasia through the Bering Land Bridge, eventually evolving into all modern horse species including modern domestic horses (Equus caballus), zebras, and wild donkeys. The true horse that remained in the Americas disappeared during the Quaternary extinction event about 10000 years ago and was not reintroduced by European colonizers until the 16th century.

The modern horse (Equus caballus) is the ultimate product of the evolution of equine animals and one of the most important domesticated animals in the development of human civilization. From a biological perspective, modern horses have highly specialized body structures: with an average shoulder height of 1.4-1.8 meters and a weight of 380-1000 kilograms, their most notable feature is the completely degenerated single hoof structure - the third toe is highly specialized to form hard keratinized hooves, while the second and fourth toes are only preserved as degenerated "split bones". This structure makes them one of the fastest running animals on land, with a maximum speed of up to 70 kilometers per hour. The head features of modern horses are significantly different from their ancestors: they have an elongated face (about 2/3 of the skull length), significantly shifted eye sockets, and are equipped with 360 degree binocular vision; All 42 teeth are high crown teeth (with a crown height of up to 10-12 centimeters), which, when combined with unique chewing movements (lateral circular motion), can effectively grind coarse fiber plants.

In terms of physiological adaptation, modern horses have evolved multiple unique mechanisms: they have the largest eyeball among mammals (with a diameter of about 5 centimeters), and their retina is rich in rod cells, giving them excellent night vision ability; The monogastric digestive system, in conjunction with the well-developed cecum (with a capacity of up to 30 liters), decomposes cellulose through microbial fermentation; The unique 'stay apparatus' musculoskeletal system enables it to stand and rest for long periods of time. Behavioral studies have shown that modern horses have complex community structures, typically forming stable groups of 5-12 horses, developing strict hierarchical systems, and communicating through rich body language (such as ear orientation, tail wagging, etc.) and 16 different neighing sounds.

From the perspective of evolutionary history, the direct ancestor of modern horses (Equus ferus) appeared about 200000 years ago and underwent multiple glacial periods of natural selection. Archaeological evidence shows that humans domesticated horses on the Eurasian grasslands as early as around 4000 BC, a process that significantly influenced the genetic diversity of horses - mitochondrial DNA of modern horses shows at least 18 maternal lineages. Nowadays, after 6000 years of artificial selection, modern horses have differentiated into over 400 varieties, ranging from the Farabela miniature horse, which is only 76 centimeters tall, to the Char Wrangler, which is 2.1 meters tall at the shoulder, exhibiting astonishing morphological diversity. As ecosystem engineers, modern horses still maintain wild populations (such as North American wild horses), playing an important role in maintaining grassland ecological balance, and their symbiotic relationship with humans has become a model of cross species collaboration.

From the forest beasts of the Eocene 55 million years ago to the galloping horses on the grasslands today, the evolution of horses perfectly embodies the extraordinary resilience exhibited by life in adapting to its environment. As revealed by Darwin's theory of evolution, this process gradually shaped the multi toed pads of early horses into efficient single hooves of modern horses through the accumulation of small heritable variations and natural selection. The cautious steps of the first horse in the forest, the flexible three toes of the middle horse in the transition zone, the high crown teeth of the grassland ancient horse to cope with rough herbivorous food, and the galloping single hooves of the real horse on the open plain - each key transformation corresponds precisely to the climate change and vegetation succession at that time. What is particularly thought-provoking is that as the new generation of grasslands expanded, the synergistic evolution of horses' teeth, limbs, and sensory systems successfully transformed them into experts in running in open areas. The domestication of horses by humans has added a dimension of cultural selection to this masterpiece of natural selection.