question: Why were animals much larger than there are today, millions of years ago?
comment: The largest animal to have ever lived is the blue whale, still around today. Many large land mammals mysteriously went extinct around the time humans start spreading across the planet, right about when humans showed up. Dinosaurs were large because some quirks of their anatomy and biology made it easier for them to get huge. For example, some had air sacs in their bones just like modern birds do, making them lighter for their size than mammals. They also had highly efficient lungs. The giant insects of the Carboniferous are the only group that was larger because of higher oxygen levels.
passages: Section::::Evolution of large body size.:In marine mammals.
   BULLET::::- The largest living primate, at up to , is the gorilla ("Gorilla beringei" and "Gorilla gorilla", with three of four subspecies being critically endangered). The extinct Malagasy sloth lemur "Archaeoindris" reached a similar size, while the extinct "Gigantopithecus blacki" of Southeast Asia is believed to have been at least two times larger. Some populations of archaic "Homo" were significantly larger on average than recent "Homo sapiens"; for example, "Homo heidelbergensis" in southern Africa may have commonly reached in height, while Neanderthals were about 30% more massive.
  
  BULLET::::- Order Rodentia
   BULLET::::- The dwarfing of animals is not explained by overkill. Numerous authors, however, have pointed out that dwarfing of animals is perfectly well explained by humans selectively harvesting the largest animals, and have provided proof that even within the 20th century numerous animal populations have reduced in average size due to human hunting.
   Section::::Evolution of large body size.:In terrestrial mammals.
   Section::::Examples.
  
  The following are some notable examples of animals often considered as megafauna (in the sense of the "large animal" definition). This list is not intended to be exhaustive:
  
  BULLET::::- Clade Synapsida
  
  BULLET::::- Class Mammalia (phylogenetically, a clade within Therapsida; see below)
  
  BULLET::::- Infraclass Metatheria
  
  BULLET::::- Order Diprotodontia
   Section::::Vertebrates.:Birds (Aves).:Cormorants and allies (Pelecaniformes).
  
  The largest-known cormorant was the spectacled cormorant of the North Pacific ("Phalacrocorax perspicillatus"), which became extinct around 1850, was larger still, averaging around and .
  
  Section::::Vertebrates.:Birds (Aves).:Bony-toothed birds (Odontopterygiformes).
   BULLET::::- Elephants are the largest living land animals. They and their relatives arose in Africa, but until recently had a nearly worldwide distribution. The African bush elephant ("Loxodonta africana") has a shoulder height of up to and weighs up to . Among recently extinct proboscideans, mammoths ("Mammuthus") were close relatives of elephants, while mastodons ("Mammut") were much more distantly related. The steppe mammoth ("M. trogontherii") is estimated to have commonly weighed around 10 tonnes, making it possibly the largest proboscid, which would make it the second largest land mammal after indricotherines.
  
  BULLET::::- Order Sirenia
   Early in the Cenozoic, following the K-Pg event, the planet was dominated by relatively small fauna, including small mammals, birds, reptiles, and amphibians. From a geological perspective, it did not take long for mammals and birds to greatly diversify in the absence of the dinosaurs that had dominated during the Mesozoic. Some flightless birds grew larger than humans. These species are sometimes referred to as "terror birds," and were formidable predators. Mammals came to occupy almost every available niche (both marine and terrestrial), and some also grew very large, attaining sizes not seen in most of today's terrestrial mammals.
   BULLET::::- The extant capybara ("Hydrochoerus hydrochaeris") of South America, the largest living rodent, weighs up to . Several recently extinct North American forms were larger: the capybara "Neochoerus pinckneyi" (another neotropic migrant) was about 40% heavier; the giant beaver ("Castoroides ohioensis") was similar. The extinct blunt-toothed giant hutia ("Amblyrhiza inundata") of several Caribbean islands may have been larger still. However, several million years ago South America harbored much more massive rodents. "Phoberomys pattersoni", known from a nearly full skeleton, probably reached . Fragmentary remains suggest that "Josephoartigasia monesi" grew to upwards of .
  
  BULLET::::- Superorder Laurasiatheria
  
  BULLET::::- Order Carnivora
   Early in the Cenozoic, following the K-Pg extinction event, most of the fauna was relatively small, and included small mammals, birds, reptiles, and amphibians. From a geological perspective, it did not take long for mammals and birds to greatly diversify in the absence of the large reptiles that had dominated during the Mesozoic. A group of avians known as the "terror birds" grew larger than the average human and were formidable predators. Mammals came to occupy almost every available niche (both marine and terrestrial), and some also grew very large, attaining sizes not seen in most of today's mammals.
   Megafauna
  
  In terrestrial zoology, megafauna (from Greek μέγας "megas" "large" and New Latin "fauna" "animal life") are large or giant animals. The most common thresholds used are weight over or (i.e., comparable or larger in mass than a human) or over a tonne, (i.e., having a mass comparable to or larger than an ox). The first of these include many species not popularly thought of as overly large, such as white-tailed deer and red kangaroo.
   BULLET::::- The largest sirenian at up to is the West Indian manatee ("Trichechus manatus"). Steller's sea cow ("Hydrodamalis gigas") was probably around five times as massive, but was exterminated by humans within 27 years of its discovery off the remote Commander Islands in 1741. In prehistoric times this sea cow also lived along the coasts of northeastern Asia and northwestern North America; it was apparently eliminated from these more accessible locations by aboriginal hunters.
  
  BULLET::::- Superorder Xenarthra
  
  BULLET::::- Order Cingulata
   Because of the small initial size of all mammals following the extinction of the non-avian dinosaurs, nonmammalian vertebrates had a roughly ten-million-year-long window of opportunity (during the Paleocene) for evolution of gigantism without much competition. During this interval, apex predator niches were often occupied by reptiles, such as terrestrial crocodilians (e.g. "Pristichampsus"), large snakes (e.g. "Titanoboa") or varanid lizards, or by flightless birds (e.g. "Paleopsilopterus" in South America). This is also the period when megafaunal flightless herbivorous gastornithid birds evolved in the Northern Hemisphere, while flightless paleognaths evolved to large size on Gondwanan land masses and Europe. Gastornithids and at least one lineage of flightless paleognath birds originated in Europe, both lineages dominating niches for large herbivores while mammals remained below 45 kg (in contrast with other landmasses like North America and Asia, which saw the earlier evolution of larger mammals) and were the largest European tetrapods in the Paleocene.
   About 500 million years ago, plants and fungi colonised the land and were soon followed by arthropods and other animals. Insects were particularly successful and even today make up the majority of animal species. Amphibians first appeared around 364 million years ago, followed by early amniotes and birds around 155 million years ago (both from "reptile"-like lineages), mammals around 129 million years ago, homininae around 10 million years ago and modern humans around 250,000 years ago. However, despite the evolution of these large animals, smaller organisms similar to the types that evolved early in this process continue to be highly successful and dominate the Earth, with the majority of both biomass and species being prokaryotes.
   BULLET::::- Anomalocarids were a group of very early legless marine arthropods that included the largest predators of the Cambrian, such as "Anomalocaris". By the early Ordovician they had evolved into giant (for the time) filter feeders, apparently in response to the proliferation of plankton during the Great Ordovician Biodiversification Event. "Aegirocassis" grew to over 2 m in length.
  
  BULLET::::- Order Eurypterida
   BULLET::::- The largest-known old world monkey, the prehistoric baboon "Dinopithecus" grew even larger than modern Mandrills, weighing as much as a grown man.
  
  BULLET::::- The largest-known new world monkey was "Protopithecus", weighing up to .
  
  BULLET::::- Some prehistoric prosimians grew to huge sizes as well. "Archaeoindris" was a lemur that lived in Madagascar and weighed , as large as a silverback gorilla. "Megaladapis" is another large extinct lemur at in length.
  
  Section::::Vertebrates.:Mammals (Mammalia).:Elephants, mammoths, and mastodons (Proboscidea).
   BULLET::::- The largest hutia are Desmarest's Hutia ("Capromys pilorides") of Cuba , a tail that is long, and weigh . The largest extinct Blunt-toothed giant hutia to have weighed between 50 and 200 kg (110 and 440 lb).
  
  BULLET::::- The largest Muroid is the Gambian pouched rat of Africa. It grows up to in total length and can weigh up to .
   Subsequent to the Cretaceous–Paleogene extinction event that eliminated the non-avian dinosaurs about Ma (million years) ago, terrestrial mammals underwent a nearly exponential increase in body size as they diversified to occupy the ecological niches left vacant. Starting from just a few kg before the event, maximum size had reached ~50 kg a few million years later, and ~750 kg by the end of the Paleocene. This trend of increasing body mass appears to level off about 40 Ma ago (in the late Eocene), suggesting that physiological or ecological constraints had been reached, after an increase in body mass of over three orders of magnitude. However, when considered from the standpoint of rate of size increase per generation, the exponential increase is found to have continued until the appearance of "Indricotherium" 30 Ma ago. (Since generation time scales with "body mass", increasing generation times with increasing size cause the log mass vs. time plot to curve downward from a linear fit.)
   Largest prehistoric animals
  
  The largest prehistoric organisms include both vertebrate and invertebrate species. Many of them are described below, along with their typical range of size (for the general dates of extinction, see the link to each). Many species mentioned might not actually be the largest representative of their clade due to the incompleteness of the fossil record and many of the sizes given are merely estimates since no complete specimen have been found. Their body mass, especially, is mostly conjecture because soft tissue was rarely fossilized. Generally the size of extinct species was subject to energetic and biomechanical constraints.
   During the American megafaunal extinction event around 12,700 years ago, 90 genera of mammals weighing over 44 kilograms became extinct. The Late Pleistocene fauna in North America included giant sloths, short-faced bears, several species of tapirs, peccaries (including the long nosed and flat-headed peccaries), the American lion, giant tortoises, "Miracinonyx" ("American cheetahs", not true cheetahs), saber-toothed cats like "Smilodon" and the scimitar cat, "Homotherium", dire wolves, saiga, camelids such as two species of now extinct llamas and "Camelops", at least two species of bison, the stag-moose, the shrub-ox and Harlan's muskox, 14 species of pronghorn (of which 13 are now extinct), horses, mammoths and mastodons, the beautiful armadillo and the giant armadillo-like "Glyptotherium", and giant beavers, as well as birds like giant condors, other teratorns and terror birds. In contrast, today the largest North American land animal is the American bison.
   BULLET::::- The largest of this order and family of prickly-skinned, small mammals is the greater moonrat ("Echinosorex gymnura"), native to the rainforests of the Malaysian Peninsula as well as Sumatra and Borneo. The maximum size of this species is over and . The moonrat is a member of the same family as hedgehogs, which are typically much smaller than the moonrat. Even larger was the giant gymnure "Deinogalerix" from Miocene Europe. It was estimated to grow larger than a house cat.
  
  Section::::Hyraxes (Hyracoidea).
   Pleistocene megafauna
  
  Pleistocene megafauna is the set of large animals that lived on Earth during the Pleistocene epoch and became extinct during the Quaternary extinction event. Megafauna are any animals with an adult body weight of over 44 kg (97 lbs).
  
  Section::::Paleoecology.
   BULLET::::- Large animals store more fat in their bodies than do medium-sized animals and this should have allowed them to compensate for extreme seasonal fluctuations in food availability.
   BULLET::::- The largest species in terms of weight is the hippopotamus ("Hippopotamus amphibius"), native to the rivers of sub-Saharan Africa. They can attain a size of , long and tall. Prehistoric hippos such as "H. gorgops" and "H. antiquus" rivaled or exceeded the modern species as the largest members of the family and order to ever exist.
  
  BULLET::::- The longest-bodied species, and tallest of all living land animals, is the giraffe ("Giraffa camelopardalis"), measuring up to tall to the top of the head, and despite being relatively slender, reaching a top weight of .
   Humans evolved from a lineage of upright-walking apes whose earliest fossils date from over . Although early members of this lineage had chimp-sized brains, about 25% as big as modern humans', there are signs of a steady increase in brain size after about . There is a long-running debate about whether "modern" humans are descendants of a single small population in Africa, which then migrated all over the world less than 200,000 years ago and replaced previous hominine species, or arose worldwide at the same time as a result of interbreeding.
  
  Section::::History of life.:Mass extinctions.
  
raw_presuppositions: Animals in the past were much larger than animals today.
raw_corrections: Some animals in the present day are still the largest of all time.