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The evolution of tetrapods began about 400 million years ago in the Devonian Period with the earliest tetrapods evolved from lobe-finned fishes. [1] Tetrapods (under the apomorphy-based definition used on this page) are categorized as animals in the biological superclass Tetrapoda, which includes all living and extinct amphibians, reptiles, birds, and mammals.
Tetrapods (animals with four limbs) 395 Amniota: Amniotes (fully terrestrial tetrapods whose eggs are "equipped with an amnion") 340 Synapsida: Proto-Mammals 308 Therapsid: Limbs beneath the body and other mammalian traits 280 Class: Mammalia: Mammals: 220 Subclass: Theria: Mammals that give birth to live young (i.e. non-egg-laying) 160 ...
The tetrapod tongue is built from muscles that once controlled gill openings. The tongue is anchored to the hyoid bone, which was once the lower half of a pair of gill bars (the second pair after the ones that evolved into jaws). [89] [90] [91] The tongue did not evolve until the gills began to disappear.
Tiktaalik, a lobe-finned fish with some anatomical features similar to early tetrapods. It has been suggested to be a transitional species between fish and tetrapods. [81] 365 Ma Acanthostega is one of the earliest vertebrates capable of walking. [82] 363 Ma By the start of the Carboniferous Period
The first tetrapods appeared in the fossil record over a period, the beginning and end of which are marked with extinction events. This lasted until the end of the Devonian 359 mya. The ancestors of all tetrapods began adapting to walking on land, their strong pectoral and pelvic fins gradually evolved into legs (see Tiktaalik). [38]
These studies do not necessarily disprove that these trackways were in fact produced by tetrapods but do at least muddy the interpretation of these trackways. Work by Niedźwiedzki et al., particularly analyzing the trackways from Poland, interprets some of the tracks as being dominated by only two limbs. [4]
After 1.5 million years ago (extinction of Paranthropus), all fossils shown are human (genus Homo). After 11,500 years ago (11.5 ka, beginning of the Holocene), all fossils shown are Homo sapiens (anatomically modern humans), illustrating recent divergence in the formation of modern human sub-populations.
As a result, in humans the muscles of the forehead (the occipitofrontalis) are only used for facial expressions. [10] Increasing brain size has also been significant in human evolution. It began to increase approximately 2.4 million years ago, but modern levels of brain size were not attained until after 500,000 years ago.