A fish didn't become a bird all in one step (or in one generation). A
fish didn't even become a reptile or an amphibian all in one step.
Instead over numerous generations of offspring a group of fish came to
have distant descendants which were amphibians, and some of those
amphibians came to have some distant descendants which were reptiles
[and other distant descendants of the amphibians were ones which had mammal-like
features]. Some of those reptiles came to have distant descendants which
were dinosaurs. Some of the dinosaurs came to have distant descendants
which walked only on the hind limbs (and thus were two legged, and had
two arms instead of two front legs) and thus were bipeds. Some of those
two legged biped dinosaurs came to have distant descendants which had
feathered dinosaurs (including feathers on the arms). Some of those
feathered two legged biped dinosaurs came to have distant descendants
which had feathered arms that were almost like wings. Some of the latter
type of feathered dinosaurs came to have distant descendants which were
birds. Scientists now consider modern birds to be avian dinosaurs, in
other words, they now consider modern birds to be a type of dinosaur -
the only type of dinosaur still in existence. Fossils of each of those categories have been found.
Fossils
were found of lobe finned fish. That kind of fish is not as well
adapted for swimming, (unlike the majority of kinds of fish), but it is
better adapted for moving between underwater plants and moving in the
muck on the bottom of oceans. Later lobe finned fish had more bones in
their paired fins, making their paired fins more like paddles with the
beginnings of toes at the ends of the paddles. Some of their distant
descendants were fish with legs. Fossils have been found for each of those kinds of fish.
Some of the distant descendants of the latter fish were the first
amphibians. The other kinds of fish continue to exist because they are
better adapted for swimming in most sections of oceans and lakes, but
the kinds with descended from the early lobe finned fish were much
better adapted (than the others) to living on land. The reptiles, birds,
and mammals (but mammals did not descend from birds, but rather from
Synapsids) are distant descendants of the early fish which had legs. See
https://evolution.berkeley.edu/what-are-evograms/the-origin-of-tetrapods/ for a good illustration of this. If you are willing to read much longer articles then see also https://en.wikipedia.org/wiki/Evolution_of_tetrapods and https://evolution-outreach.biomedcentral.com/articles/10.1007/s12052-009-0119-2 .
It
was asked why are still fish if birds evolved from fish. The answer is
because fish are better adapted to water than birds, though some birds
do spend some their time swimming under water. Certain types animals
live in certain environments and certain other types of animals live in
other environments.
Humans evolved from ancient species of apes
(not modern day species of apes). Regarding why do apes still exist
today if humans evolved from apes, it is because existing apes are well
suited for their jungle environment, which includes spending time
climbing trees and being on tree limbs (in some cases even sleeping on
tree limbs). Some ape species are also well adapted for swinging from
branches, unlike humans. The orangutans and gibbons swing from branches
and have long arms which are well suited for that.
TD, you provided a great line up of skulls but I don't think the depicted photos of the skulls are to the same scale. The oldest skulls are much smaller than modern human skulls, but the photos make it look like they all have the same height from the bottom of the chip to the top of the cranium. If the photos were depicted to the same scale, the oldest skulls would be noticeably much smaller than modern human skulls.
