Hi Gumboot,
Dolphins and sharks look fairly similar because of a phenomenon known as parallel evolution. In essence the fact that they live in the ocean means that swimming is of paramount importance to them, and to swim quickly there are certain hydrodynamic proportions / shapes that work better than others. Any mutation, say for greater intelligence, that took them away from this streamlined (one could say fish like) shape, would work against them.
Except it's not parallel evolution. Dolphins evolved on land for millions of years, and only then entered the water, during which time sharks remained relatively unchanged.
A shark and a snapper feature parallel evolution. Not a dolphin. And a dolphin has vastly higher intelligence than sharks or snapper. Why? Because of all that time as a mammal on land, in circumstances that enabled higher intelligence.
Quadropedia is a useful format, but there's no reason to assume that it is the only successful body format, or that intelligence will arise from that particular form. Six legs (insects), eight legs (arachnids), and more legs (centipedes etc) are also successfuly body formats.
This is true, but only in very small animals.
The advantage of higher numbers of legs is stability and mobility in very uneven terrain. For tiny animals such as insects and spiders, even the smallest ground variations act as enormous obstacles, so being able to maneuver around sheer walls and difficult terrain is vital.
Once you get to the size of larger animals, the advantage offered by more legs is countered by lost efficiency because all of those legs require energy to function. The likely reason all limbed terrestrial vertebrates are tetrapods is because the earliest limbs (pectoral fins, in primitive fish) acted to provide steering and to provide buoyancy by generating lift. Having multiple ranks of fins would not offer any significant benefit as the secondary and tertiary pairs of fins would be in stalled water behind the front fins, and therefore incapable of providing an advantage in maneuvering or lift generation.
Once vertebrates looked to move onto land the most efficient and expedient way of this occurring is through the addition of a second pair of limbs. As a result all large land animals (and their descendants) are tetrapods.
Arthropods aren't capable of developing to the size necessary for dominating the land and becoming a super-predator, because they're invertebrates. Again we see evolution forcing development along a narrow range of options.
Once you get into flat open terrain (which is what motivates the bipedal arrangement that enables tool use and leads to brain development) the advantages of having more than four legs become even less significant, and having less limbs actually starts to be an advantage. Even if it were possible to come up with a scenario where, say, large six-limbed terrestrial vertebrate evolved (in all honesty I am pretty confidence it would be evolutionarily improbable), you would probably find that they would have lost one set of limbs long before becoming bipedal. You'd still end up with a species that had a basically humanoid appearance.
Intelligence will arise in a species when it becomes an evolutionary advantage for that species to have it. Then those that are smarter for whatever mutation they have will be selected for. As long as that intelligence doesn't come with a disadvantage as well.
And there are many reasons why it's advantageous for a creature to be smarter. To recognise foes and prey, to work out how to hide, to make better use of the sensory data that they have, to cooperate in hunting, to rise in social orders etc. These reasons are not in any way limited to quadropeds.
You're approaching this argument entirely backwards. It's not really a question of which traits offer an advantage. Given how beneficial higher intelligence is, it goes without saying that every single species on this planet would benefit enormously from higher intelligence. Yet exactly one species has developed it.
Evidence increasingly suggests that intelligence was the last major characteristic humans picked up, and it seems more and more likely that our higher level intelligence developed long after we'd essentially placed ourselves outside the evolutionary game by becoming the planet's top super-predator.
The fossil record indicates humans were bipedal and using tools some 4 million years ago, yet only developed the first stages of a proto-language between 2.3 and 0.6 million years ago, and only developed
true language about 100,000 years ago, by which time man was anatomically speaking no different than man today. Modern behavioral man didn't appear until around 50,000 years ago.
In other words, we only achieved higher intelligence
because we were
H. sapiens sapiens. Our intelligence was a direct result of us occupying a sort of evolutionary "perfect storm" where a wide variety of factors came together to enable development of higher intelligence. And intelligence being what it is, once it gets to a point it becomes self-sustaining, leading to increasingly accelerated intelligence.
So while every other species on this planet would benefit enormously from having our intelligence, none of them will ever evolve it, because none of them have found that "perfect storm" spot where higher intelligence is able to take seed.