Pentadactyl
Limb
Anatomists realised that some different species of animals have similar structures but have different functions for them, this is a homologous structure, for example the pentadactyl limb. The pentadactyl limb can be found in animals such as humans, bats and whales. Penta = 5, Dactyl = Fingers. The number of bones and the size of the bones in a pentadactyl limb can vary but the format of each pentadactyl limb has the same original format no matter what the function is. This homologous structure is proof that the organisms have common ancestors, each pentadactyl limb is adapted differently to other species helping each of them to survive in their habitats. Darwin noticed how widespread the pentadactyl limb was when he wrote On the Origin of Species.
'What could be more curious than that the hand of man formed for grasping, that of a mole, for digging, the leg of a horse, the paddle of a porpoise and the wing of a bat, should all be constructed on the same pattern and should include similar bones and in the same relative positions?' - Darwin. Fig 11.
The pentadactyl limb is common to most tetrapod's, this is evidence to humans relation to amphibians, reptiles and many other mammals.
Anatomists realised that some different species of animals have similar structures but have different functions for them, this is a homologous structure, for example the pentadactyl limb. The pentadactyl limb can be found in animals such as humans, bats and whales. Penta = 5, Dactyl = Fingers. The number of bones and the size of the bones in a pentadactyl limb can vary but the format of each pentadactyl limb has the same original format no matter what the function is. This homologous structure is proof that the organisms have common ancestors, each pentadactyl limb is adapted differently to other species helping each of them to survive in their habitats. Darwin noticed how widespread the pentadactyl limb was when he wrote On the Origin of Species.
'What could be more curious than that the hand of man formed for grasping, that of a mole, for digging, the leg of a horse, the paddle of a porpoise and the wing of a bat, should all be constructed on the same pattern and should include similar bones and in the same relative positions?' - Darwin. Fig 11.
The pentadactyl limb is common to most tetrapod's, this is evidence to humans relation to amphibians, reptiles and many other mammals.
The Bat
The Bat is well adapted to the environment which it lives in, one of the main adaptations is the wing which they use to fly, this is their pentadactyl limb. It is however extremely difficult to tell much about the evolution of the bat using fossils, their delicate structures and small bones are not often found intact with any use to us to research as they are too fragile. Bats elongates bones have evolved to help them to support the wings. Natural selection has altered and evolved animals, specifically flying animals like the bat. Functional wings and flight are one of the main characteristics. The skeleton of the bat is homologous with structures in the forelimbs of many other mammals so there is no evidence to prove that their pentadactyl limb (the wing) did not evolve as a modification from their previous ancestors. The wing skeleton which is closest to the body is the humerus, it is long and thin compared to the majority of other mammals but the attachment of the muscles is very similar to most other mammals. Attached to the humerus is the radius and the ulna. The radius of the bat is also very long and thin but it still plays a key role in supporting the wing. The ulna has been massively reduced in size in comparative to the other bones and it is fused with the radius. The wrist area of the bat is similar to most other mammals but there is a difference, the wrist region of the bat is less flexible, this is because it is specialised to support motions associated with flying. All bats have thumbs, these sit along the edge of the wing, the thumb also has a claw which is there for fighting, climbing and handling food. These thumbs can vary in size, this often depends on their feeding habits and weather they include a substantial amount of crawling or not. The rest of the digits which they have in their pentadactyl limb are for supporting the wing, each of these has a basal metacarpal and between 1 and 3 phalanges. It is believed that they evolved from animals which would climb up a tree into a high area and then glide, this is believed due to the fact gliders wing membrane stretches down across the hind limbs as it does in bats and there is no other known animal which has this exact structure.
The Bat is well adapted to the environment which it lives in, one of the main adaptations is the wing which they use to fly, this is their pentadactyl limb. It is however extremely difficult to tell much about the evolution of the bat using fossils, their delicate structures and small bones are not often found intact with any use to us to research as they are too fragile. Bats elongates bones have evolved to help them to support the wings. Natural selection has altered and evolved animals, specifically flying animals like the bat. Functional wings and flight are one of the main characteristics. The skeleton of the bat is homologous with structures in the forelimbs of many other mammals so there is no evidence to prove that their pentadactyl limb (the wing) did not evolve as a modification from their previous ancestors. The wing skeleton which is closest to the body is the humerus, it is long and thin compared to the majority of other mammals but the attachment of the muscles is very similar to most other mammals. Attached to the humerus is the radius and the ulna. The radius of the bat is also very long and thin but it still plays a key role in supporting the wing. The ulna has been massively reduced in size in comparative to the other bones and it is fused with the radius. The wrist area of the bat is similar to most other mammals but there is a difference, the wrist region of the bat is less flexible, this is because it is specialised to support motions associated with flying. All bats have thumbs, these sit along the edge of the wing, the thumb also has a claw which is there for fighting, climbing and handling food. These thumbs can vary in size, this often depends on their feeding habits and weather they include a substantial amount of crawling or not. The rest of the digits which they have in their pentadactyl limb are for supporting the wing, each of these has a basal metacarpal and between 1 and 3 phalanges. It is believed that they evolved from animals which would climb up a tree into a high area and then glide, this is believed due to the fact gliders wing membrane stretches down across the hind limbs as it does in bats and there is no other known animal which has this exact structure.
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Figure 13 - This video shows just how well the bats pentadactyl limb has evolved and how it is now able to use it to travel across distances effectively.
The Horse
The horse also has a pentadactyl limb although it may not look like it to the naked eye anymore, this is because it has evolved but primarily it still has the same structure. The horse still has remains of other 'fingers' present but not is only seen with one 'finger'. The earliest ancestors of horses today (around 50 million years ago) were much smaller than they are today and the feet in contact with the ground had 3 toes as well as the fourth toe on the front legs. This is presumed to be a development from the standard pentadactyl limb which most mammals have. Later the middle toe developed to be larger and the two outside toes were reduced. The development of these larger limbs and now having hooves means that they can survive much more successfully in grassland habitats, this allows horses to spread out over a larger area as they evolved. The structure of the horses hoof is often used as 'concrete evidence' of evolution. From the earliest fossils of horses we discovered two of the five digits had already become redundant. The long legs and the hoof of the horses evolved as it meant that they were able to travel further and do it faster. A horse is effectively running on the equivalent of our middle finger. The hooves of a horse are now vital for carrying body weight, the front hooves can carry between 40 and 60 percent of the horses weight. The foot of the horse slowly adapted to become a one toed animal and also adapted its hoof so that there is a 'frog' which helps to pump blood around the body of the horse. The splint bones are now all that is left of the second and fourth metatarsal at the back of the leg and metacarpal at the front of the leg. Now only the canon bone which has evolved from the third metatarsal and the metacarpal has kept its functional form as a toe. The horses pentadactyl limb adapted primarily for running over distances and increasing the pace in which they do this. It is suggested that previously horses were about as small as a fox with five digits on each of their feet but due to natural selection and avoiding extinction they have become the hooved horse which we know today.
The horse also has a pentadactyl limb although it may not look like it to the naked eye anymore, this is because it has evolved but primarily it still has the same structure. The horse still has remains of other 'fingers' present but not is only seen with one 'finger'. The earliest ancestors of horses today (around 50 million years ago) were much smaller than they are today and the feet in contact with the ground had 3 toes as well as the fourth toe on the front legs. This is presumed to be a development from the standard pentadactyl limb which most mammals have. Later the middle toe developed to be larger and the two outside toes were reduced. The development of these larger limbs and now having hooves means that they can survive much more successfully in grassland habitats, this allows horses to spread out over a larger area as they evolved. The structure of the horses hoof is often used as 'concrete evidence' of evolution. From the earliest fossils of horses we discovered two of the five digits had already become redundant. The long legs and the hoof of the horses evolved as it meant that they were able to travel further and do it faster. A horse is effectively running on the equivalent of our middle finger. The hooves of a horse are now vital for carrying body weight, the front hooves can carry between 40 and 60 percent of the horses weight. The foot of the horse slowly adapted to become a one toed animal and also adapted its hoof so that there is a 'frog' which helps to pump blood around the body of the horse. The splint bones are now all that is left of the second and fourth metatarsal at the back of the leg and metacarpal at the front of the leg. Now only the canon bone which has evolved from the third metatarsal and the metacarpal has kept its functional form as a toe. The horses pentadactyl limb adapted primarily for running over distances and increasing the pace in which they do this. It is suggested that previously horses were about as small as a fox with five digits on each of their feet but due to natural selection and avoiding extinction they have become the hooved horse which we know today.
Figure 15 - This video explains the evolution of horses and their pentadactyl limb along with other adaptations from 55 million years ago.