Prehistoric Earth was home to many gigantic creatures, from colossal dinosaurs to enormous woolly mammoths. This ancient gigantism prompts curiosity about the underlying environmental and biological factors that allowed such immense sizes to evolve and thrive.
Atmospheric Conditions
Higher oxygen levels in Earth’s atmosphere contributed to the immense size of some ancient animals. During the Carboniferous period, approximately 300 million years ago, atmospheric oxygen concentrations reached a peak of up to 35%, significantly higher than the current level of about 21%.
Insects and other arthropods breathe through tracheae, which don’t grow proportionally with their bodies. Higher oxygen allowed more efficient diffusion, supporting larger bodies. The fossil record includes giant insects like the griffinfly, an extinct relative of the dragonfly, with a wingspan of up to two feet.
Climate and Resource Availability
Abundant resources and a favorable climate also fostered large animal sizes. The Mesozoic Era, or “Age of Dinosaurs,” had generally warm temperatures with less extreme variations than today. This stable climate led to lush global vegetation.
During periods like the Jurassic, widespread humid conditions supported extensive forests dominated by gymnosperms. This environment provided a consistent, plentiful food supply, removing a major constraint on animal growth. The continuous availability of energy-rich plant matter allowed herbivorous animals to grow to enormous sizes, which in turn supported large carnivorous predators within a rich food web.
Ecological and Evolutionary Pressures
Ecological and evolutionary pressures also favored gigantism. A larger body size often conferred advantages in defense against predators or for competing with rivals for mates and resources. In some environments, the sheer scale of ancient ecosystems, with vast, open ecological niches, allowed animals to grow without immediate limitations on space or resources.
Large body size also offered thermoregulation benefits, known as gigantothermy. Animals with greater mass have a smaller surface area-to-volume ratio, meaning they lose heat more slowly than smaller animals. This characteristic helped maintain a stable internal body temperature, particularly in environments where temperatures fluctuated or were consistently cool, making large sizes energetically favorable.
The abundance of prey in past ecosystems allowed for the evolution of immense carnivores that could effectively hunt large prey.
Why Modern Animals Are Smaller
Modern animals are smaller due to significant environmental and ecological shifts. A primary reason is dramatic changes in global climate patterns. The onset of various ice ages, particularly during the late Cenozoic, led to widespread cooling and the transformation of expansive habitats, such as open tundras, into forests or fragmented landscapes. These new conditions favored smaller animals better adapted to colder temperatures or fragmented forest environments.
Declining atmospheric oxygen levels from their Carboniferous peaks also played a part, particularly limiting the maximum size achievable by insects. The rise of humans has also been a major factor in the reduction of animal size, especially among megafauna. Human hunting, often referred to as the “overkill hypothesis,” is a widely debated but significant contributor to the extinction of many large animal species during the late Pleistocene.
Human activities also caused widespread habitat fragmentation, breaking continuous natural areas into smaller patches. This fragmentation disproportionately affects larger animals, which typically require extensive ranges to find food, mates, and suitable living conditions. Over the last 300,000 years, since the emergence of modern humans, there has been a noticeable trend of size-biased extinction, resulting in a significant decrease in the average body size of mammals globally.