Homologous and Analogous Strucutres

1a) The trait I have chosen is the mammalian forelimb. Two species in particular would be human beings, who are land-dwelling, upright bipeds, and sea lions, which are a marine, and thus swimmer, species (though they can surface for periods of time).

1b) The human forelimb has an extended humerus as compared to the sea lion, as well as a longer ulna. Carpal structure is also similar, but the metacarpals and phalanges of humans are relatively short, however separated. The sea lion metacarpals and phalanges appear to be longer but connected and to function more as a whole unit, as opposed to the human digits that can function independently of the others.

These traits exhibit differences between the two species because of the environments in which the species evolved. Though there is a similar ancestor, human beings ultimately ended up on land and walking upright led us to find uses for our hands. Sea lions use their forelimbs are flippers that propel them through water. The different environmental pressures drove the two different forelimbs to evolve to serve two different, but similar, purposes.

 

1c) A common ancestor between sea lions and humans would have been a placental mammal that was probably a quadruped that lived either by land or sea. The mammalian forelimb was present in some of the oldest known mammals (up to around 200 mya) and was likely a feature that evolved differently after different speciation events. The latest common ancestor between sea lions and humans would have been around 80-100 mya.

2a) The analogous trait I have chosen is the eye. Two species that posses eyes but have no homologous relation are humans and octopuses. Humans, again, are land dwelling creatures, octopi are marine creatures.

2b) Both eye structures have an eyelid, a cornea, a pupil, and a lens that focuses the light onto the retinal cells coating the rear of the eye. There are also receptor cells, also on the rear of the eye, that collect and send this information via the optic nerve, again at the rear of the eye structure. There are similarities between these structures because both of these species adapted to the benefits of being able to see (process light information). Primitive photosensitive structures on ancestors of either species likely mutated slowly in order to see clearly (somewhat) in varying conditions.

2c) Given that the last common ancestor between humans and octopuses lived hundreds of millions of years ago, it is unlikely that a common ancestor would have had eyes. This time period contains some of the most primitive animals, and eye structures are some of the most complex evolved structures known to biology. It is possible that basic photosensitive structures existed on the earliest animals, but nothing close to an eye.