Phylogenic evolution of the knee

During the Late Devonian period (400 million years ago), Sarcopterygii were the first lobe-finned fish to emerge from the aquatic environment and later differentiate into tetrapods with the appearance of Tiktaalik and Ichthyostega (340 million years ago). In these species, the caudal fin was replaced by a hind limb with a bicondylar femur articulating with the tibia and fibula. There is no evidence of the existence of a patella at this stage.The first amphibian was Eryops, which appeared around 300 million years ago and possessed a fully developed knee. It later differentiated into a reptilian branch, some species of which had an osseous patella, and from which existing species remain today: the crocodile, in which the femur is broad and articulates with the tibia and fibula but without a patella or menisci; the lizard, in which a poorly ossified patella and rudimentary menisci appear; and birds, which—like the emu—display an ossified patella and a deeply grooved trochlea.The other branch derived from Eryops gave rise to mammals or primitive mammals and to the first anthropoid primates around 39 million years ago. These are now known to have migrated from Asia to Africa across a primitive ocean, the Tethys, giving rise to three separate groups: the Platyrrhines, at the origin of New World monkeys; the Catarrhines, which later split in Africa into Cercopithecoidea and Hominoidea (tailless apes) around 6 million years ago.
Knee evolution is marked by widening of the tibia leading to exclusion of the fibula, and by the development of the patella. These hominoids, arboreal quadrupeds, subsequently diverged into three families: the Hylobatidae, from which climbing quadrupedal gibbons originated; the Pongidae, which evolved into suspensory quadrupedal orangutans; and the Hominidae, at the origin of Paninae and Homininae.The Paninae evolved into gorillas and chimpanzees, which remain climbing quadrupeds with the ability to adopt an erect posture. The Homininae gave rise to australopithecines adapted to a form of bipedalism and to the genus Homo, which became exclusively bipedal. The common ancestor of the two subfamilies is unknown; the oldest fossil discovered to date is Toumaï (Sahelanthropus tchadensis), dated to 7 million years ago, whose postcranial skeleton attests to a form of bipedalism. In pongids and panins, upright posture occurs only with hip and knee flexion, and full vertical alignment is impossible due to the lumbar spine being wedged between the iliac wings, preventing lordosis. This vertical alignment became possible in australopithecines owing to the appearance of a sixth lumbar vertebra.The knee of great apes shows varus alignment, which disappears in australopithecines, although with significant intercondylar distance related to pelvic width. Femoral valgus appears predominantly in the genus Homo. There is also a modification of the condylar profiles, which are almost circular in great apes with a very extensive trochlear surface, and more ellipsoid in the genus Homo with a reduced trochlear surface.Thus, the primate knee is patella-dependent, whereas the human knee is tibia-dependent. Meniscal evolution is also observed: the lateral meniscus in gorillas and chimpanzees is free of any capsular attachment, with a common anterior and posterior insertion, unlike the human meniscus. Finally, the anterior cruciate ligament (ACL) is highly developed in these primates, comprising five bundles.
Throughout this long evolutionary process, there is a marked modification of trochlear shape according to modes of locomotion, which is also reflected in the corresponding patellae. This likely explains the frequent trochlear dysplasias encountered in clinical practice and the high frequency of femoropatellar instability.