Many animals in the distant past were larger mainly because their environments, climates, and ecosystems favored big bodies, and because evolution had a lot of time and “ecological room” to produce giants.

Big bodies had survival advantages

In many prehistoric ecosystems, being large was simply a good strategy.

  • Large herbivores could reach more food (like high foliage) and process huge amounts of plants efficiently.
  • Big bodies made animals harder to hunt, so size offered protection from predators.
  • Once herbivores became large, predators also evolved larger bodies to tackle them, creating an “arms race” in size.

Ancient climates and resources

Prehistoric worlds often had climates and landscapes that supported very large animals.

  • Long periods of warm, relatively stable climate with vast, lush vegetation meant huge amounts of plant biomass to fuel big herbivores.
  • More extensive, undeveloped land meant less fragmentation and more continuous habitat, so giants could roam and feed over large areas.
  • In some periods there was much more atmospheric carbon dioxide, which boosted plant growth and allowed dense, productive ecosystems.

Oxygen and body size (especially insects)

Oxygen played a role, but in a more limited and nuanced way than popular explanations suggest.

  • During the Carboniferous period, higher oxygen levels allowed some insects and arthropods to grow very large because their passive breathing system works better when oxygen is abundant.
  • For dinosaurs and most later vertebrates, oxygen levels were not consistently so much higher than today, so physiology (efficient lungs, air sacs, lighter skeletons) mattered more than oxygen alone.

Special dinosaur “engineering”

Many giant dinosaurs had anatomical features that made huge size more manageable.

  • Hollow, air-filled bones reduced weight while still supporting massive bodies, similar to modern birds.
  • Very efficient lungs and air-sac systems helped move large volumes of air and supply enough oxygen to their bodies.
  • Long necks in sauropods let them stand in one place and sweep wide areas of vegetation without spending much energy moving.

Why animals aren’t usually that big today

Modern conditions make extreme giants rarer and riskier.

  • After mass extinctions (like the end-Cretaceous event that wiped out non-bird dinosaurs), many giant lineages disappeared and later ecosystems were repopulated by smaller, faster-reproducing animals.
  • Large animals need huge, stable habitats and lots of food; human-driven habitat loss, hunting, and climate change make it hard for very large land animals to persist.
  • There are still giants by historical standards—like blue whales, the largest animals ever known—but on land, ecological pressures now favor more moderate body sizes overall.

Information gathered from public forums or data available on the internet and portrayed here.