The answer is homoplasy, which is another term for convergent evolution. Humans and squid evolved the eye independently, which explains the differences in structure despite similar function.

The air had plenty of CO2 in it when the move to land was made. The biggest problems that these organisms faced was (1) retention of water because water wasn't as readily available on land and (2) supporting itself against gravity because it could no longer rely on the water to support its structure, as air can't support an organism like water can.

The abnormal number of chromosomes in a cell is referred to as aneuploidy. This event occurs when the chromosomes do not separate properly in cell division. Only one chromosome is affected, leaving one too many or too few copies.

In contrast, polyploidy refers to organisms that have more than two copies of every chromosome. For example, triploid organisms have three copies of every chromosome. This differs from trisomy, in which an organism carries three copies of a single chromosome (as described in the question).

The endosymbiotic theory states than a large ancestral prokaryotic cell engulfed an aerobic prokaryotic cell. Eventually, the engulfed cell formed a relationship with the host cell, becoming an endosymbiont (a cell living within another cell).  Host cells and their endosymbionts merged into a single organism: a eukaryotic cell with mitochondria and chloroplasts. Proof for this theory is found in the fact that both chloroplasts and mitochondria have two membranes and their own ribosomes and circular DNA molecules, similar to those found in prokaryotic cells. DNA within these organelles allows for independent synthesis of proteins and ribosomes. These organelles also grow and reproduce independently within the cell.

Duplication is a type of mutation in which a gene is duplicated within a genome. This creates the potential for one of the duplicates to evolve a new function, with the other copy retaining the original function. Deletions, inversions, and translocations are potentially harmful to the native function of a gene, whereas duplications of entire genes yields no potential for loss-of-function mutations.

The Miller-Urey experiment was performed in the 1950s to test if Earth’s early atmosphere was capable of synthesizing organic compounds needed to support the origin of life. In the experiment, molecules believed to be present in Earth’s early atmosphere were placed in a closed system and electricity was run through it. This resulted in the production of amino acids and organic compounds; therefore, this supported the hypothesis that the conditions on early Earth could have aided in the synthesis of organic compounds.

There are many theories regarding the origins of life on Earth, many of which are scientifically supported and it is believed that several may have occurred in tandem. The most notable of these theories is the Big Bang theory others include: the arrival of organic compounds from meteoroids, the synthesis of compounds from Earth’s atmosphere, and the RNA world hypothesis.

According to the RNA world hypothesis, RNA was a precursor for all other molecules the origins of life on early Earth. The characteristics of RNA support the hypothesis that RNA may be the precursor to current life. These characteristics include the ability of RNA to self-replicate, catalyze simple reactions, and store information.

The current hypothesis for the evolution of eukaryotes is the theory of endosymbiosis. In this theory, one cell engulfs another cell, creating a symbiotic relationship between the host cell and engulfed cell. The internal endosymbiotic cell is inherited through the generations and is the precursor to organelles. 

Evolution occurs slowly over time as small changes accumulate in the DNA of a given population. Evolution is not a blueprint for perfection and does not create a hierarchy of developed and non-developed species.