Evolution of Knees

The knee is a complicated wonder of design. The joint is a product of evolution from the ancestor we shared with the chimpanzee to allow for bipedalism. Scientists strongly believe that the knee’s evolution is one of the factors responsible for our success and ability to thrive as a species. However, even with the impressive complexity of the design visible in the human knee, it is essential to point out that it has its flaws too. The main defect manifests itself in the form of pain, as seen in osteoarthritis. 

There is no perfect human knee model, even though the similarities in knees are noticed. That said, it should be highlighted that humans are the only species that possess both biped and plantigrade features. The knee joint is the largest in the body, and it is also the most vulnerable one. 

The massive synovial membrane, semilunar cartilages, covering patella and the cruciate ligaments are all parts that have been positioned to offer stability and protection. However, it is precisely these several parts that serve as the sites of knee pathologies.

Evolutionary Phases of the Knees

Initially, all the animals, including the vertebrates, all dwelled inside water. Locomotion was then achieved by sending waves of contraction along the muscular portions of their bodies. Evolution set in, and the fins were projections of the skin or body wall, and these worked in ensuring balance and took part in the steering of tapered bodies. 

With time, the water bodies became shallower in some places; some fishes adapted to the swamp life and were breathing in the air while using the fins as the paddles. Limbs were used for crawling along the banks of the muddy puddles or rivers. In looking for more food, the animals climbed greater heights, and over millions of years, they started developing limbs as seen in lizards and turtles, which were able to lift their bellies off the ground. 

The higher the animals climbed, the longer the limbs grew, which led to an increase in the joints’ sophistication. In the end, they learned to run swiftly on the ground, climb the trees, and even swing from one branch to the other. Deserts, jungles, forests, swamps, mesas, prairies, mountains, and climate factors triggered changes and modified the body and limb. 

Nature kept on working on these features, and there were noticeable modifications and improvements. This way, the bodies of the animals that have evolved recently ended up having several structures. The summary here is that the evolution of knees came about due to the functional necessity that was needed for survival itself. The simplest present-day vertebrate with a knee joint is the amphibian, and the salamander is a good illustration. 

Around about 1.8 million years ago, the genus Homo dispersed out of Africa. This dispersal demonstrated changes in the behavior, diet, and efficiency of bipedal locomotion. Even though several morphological features of the ankle and knee joints are components of features connected to regular bipedal walking, the period and genetic details of these changes in structure and function are still subjects of debate. 

The analysis of the timing of ankle and knee joint evolution is done using geometric morphometric methods which are then applied to 3D models of the distal and proximal tibiae of fossil hominins, extant great apes, and the Holocene Homo sapiens. 

The evolution is marked with a pair of curve semi landmarks and landmarks as seen on all the specimens used. As some of the fossils were not complete, researchers had to make use of digital reconstructions to assess the missing semi landmarks and landmarks. A derived proximal tibia (the knee) shares similarity with the ones seen in Homo sapiens and it evolved with the early Homo around two million years ago. 

On the other hand, derived features of the distal tibia appeared later on the evolutionary scale, with the advent of Homo erectus. The outcome points to a separation of the distal and proximal tibia, suggesting an array of functions. This led to the evolution of selective pressure at different points on the joints. Dispersals over longer distances involved the Dmanisi hominins to places like Georgia around 1.8 million years ago. At around the same time, there was a similar fanning out of Homo erectus to the east and Southeast Asian regions, specifically about 1.6 million years ago. 

These dispersals were triggered by the evolution of a structurally-derived knee unit which is similar to the recent knees in humans and an ankle that was structurally primitive. 

The asymmetrical and complicated design of the human knee is one with origin going back to ancient times. This design’s specific features have been traced to have been in place since over 300 million years ago. Observation has shown that the knees of a vast majority of tetrapods groups show the same morphological properties. These include the menisci, bicondyles of the femur’s distal portion, collateral ligaments placed asymmetrically, and the intra-articular ligaments. 

Apart from the similarities in morphology, there are also similarities in the functional properties. This is exemplified by the femur-tibia contact locomotion’s dynamic axis in the rear of the tibia when flexed. It is concluded that the joint design and function seen in tetrapods is a pointer to the common root of the kinematic principles.  

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