Earth’s history features creatures of immense proportions, often called megafauna or exhibiting gigantism, inspiring curiosity about their past existence. Megafauna are typically defined as animals exceeding 44 kilograms (97 pounds), though some definitions extend to over 1,000 kilograms (2,200 pounds) for megaherbivores. This raises a central question: what factors allowed so many animals in Earth’s past to attain such impressive sizes?
Atmospheric Oxygen Levels
Higher atmospheric oxygen concentrations during certain geological periods facilitated the growth of exceptionally large animals, particularly invertebrates. During the Carboniferous and Permian eras, around 300 million years ago, oxygen levels reached approximately 35%, significantly higher than the current 21%. This elevated oxygen directly benefited insects and other arthropods, which respire through a system of tubes called tracheae that deliver oxygen directly to their tissues.
Insects’ tracheal systems become less efficient at larger sizes as oxygen must diffuse over longer distances. More atmospheric oxygen relaxed this physiological constraint, allowing for the evolution of giant forms like griffinflies, extinct relatives of dragonflies, with wingspans up to 71 centimeters (over 2 feet). While the direct impact of high oxygen on the gigantism of all large animals, such as dinosaurs, is debated, its role in enabling the scale of ancient invertebrates is widely recognized.
Climatic Stability and Resource Abundance
Earth’s stable and favorable climate for extended periods also supported the evolution of gigantism. Past geological eras featured warmer and more consistent global temperatures without the dramatic fluctuations seen more recently. Such stable conditions reduced the energy animals needed for temperature regulation, freeing up metabolic resources for growth.
These prolonged periods of warmth fostered lush environments, leading to abundant food resources that sustained massive body sizes. Extensive grasslands and forests provided a continuous supply of vegetation for large herbivores, which formed the base of the food web for carnivores. A consistent food supply allowed animals to grow continuously over longer lifespans, reaching sizes challenging to maintain in less stable or resource-scarce environments today.
Evolutionary Advantages of Large Size
Large size offered several evolutionary advantages. Increased size provided an effective defense against predators, as a larger animal is more difficult for smaller predators to overpower. This deterrence meant large animals faced fewer threats, improving their survival and reproduction.
For many large ectothermic (cold-blooded) animals, size also conferred a thermoregulatory benefit known as gigantothermy. Larger bodies have a lower surface-area-to-volume ratio, meaning they lose heat more slowly and maintain a more stable internal temperature. This allowed some ancient ectotherms to function effectively in varying temperatures, similar to endothermic (warm-blooded) animals.
Large animals could also access resources unavailable to smaller counterparts, such as reaching higher vegetation or traveling greater distances for food. Increased size often provided advantages in competition for mates or territory, contributing to reproductive success. Large ectotherms also exhibit slower metabolic rates per unit of mass, requiring less frequent feeding compared to smaller animals.
The Decline of Megafauna and Gigantism
Despite evolutionary advantages and favorable conditions, widespread gigantism eventually waned, particularly towards the end of the Pleistocene epoch. A primary driver was increasing climate variability, including significant cooling trends and numerous ice ages. These shifts reduced lush habitats and created resource scarcity, making it harder for massive animals to find sufficient food and adapt to rapidly changing environments.
The evolution of new species and increased competition for dwindling resources also pressured megafauna. The arrival and expansion of humans introduced a new factor. Overhunting by early human populations, alongside habitat destruction and altered fire regimes, played a substantial role in the extinction of many megafaunal species, especially in continents like North America, South America, and Australia where human arrival coincided with rapid extinctions. While the exact interplay between climate change and human impact remains a subject of ongoing research, human activity is often considered a tipping point that pushed vulnerable populations towards extinction.