Koopa and I have learned much on our journey so far together. He tells me when he doesn't like something in his own turtle-y way. When he was very small he wouldn't eat any of the grasses or food I provided and wasn't growing, and so I caved as a mommy and fed him delicious lettuces from romaine, spinach, kale and beyond. He was a happy baby, and was eating, but then I noticed his growth wasn't right, he began to pyramid (pyramiding is where instead of the shell growing uniformly flat around the edges of each scute, each scute begins to grow upward making each scute look like a little pyramid). Let me define scute as well, when you look at a turtles shell it is made up of a bunch of units, each of these units is a scute. Scutes are the outermost form of armor on a turtles shell. The ribs form the base of the shell on the inside (by the turtles guts), they are fused to each other creating a solid bony box that the head, arms, legs and tail poke out of. The scutes are on the outside of this bony box. An interesting anatomical note is that the shoulder blades of a turtle are INSIDE the ribcage, swing your arms around a little and think about how weird that is - our shoulder blades are on the outside of our ribs. Turtles have accomplished this through many minor changes in their evolutionary development that I will write about in more detail at a later date for those of you who are interested (it's super totally amazingly cool but too detailed for this post). Koopa is now back to a diet he would encounter in his native element.
The answer to that question is no. Tortoises have each converged on an exclusively terrestrial lifestyle from many different independant turtle families. A sulcata is not more closely related to another tortoise because they're both terrestrial - they could be related because they share common ancestors in the same region - but is most likely more closely related to a turtle species in a different habitat in that same region, which could be extant or extinct. The key to surviving is to capitalize on individual mutations that you posses and exploit them in a habitat with less competition for resources. This is what tortoises have done, they acquired traits like armored limbs for burrowing, keen senses of smell and sensitivity to color, and a lengthened intestinal tract for digesting plant matter versus a high protein diet which enabled them to survive in new environments where there are less turtles competing for the same resources.
I want to note that tortoises did not choose to acquire these traits because they thought they would be good to have to survive in harsh environments, these characters were acquired randomly through genetic mutations in DNA that created many different outcomes. Some proto-tortioses did not get dealt the right genetic hand to succeed in a hot, dry, protein deficient environment and they died - but a few did get the right hand, completely randomly in a random order over huge lengths of time and succeeded.
Another note is that gigantisism in tortoises is not commonplace, there are factors selecting for gigantisism or else there are a relaxation of factors selecting for small size. Sulcatas are likely giant because once an individual reaches a certain size a lion can not break the shell open with its jaws, so gigantisism has been selected for as a tool for surviving with large predators. Galapagos tortoises are the other example, there was no selection for staying small to avoid getting spotted and eaten by predators, because there are no predators like large birds of prey or giant cats, so selection was relaxed and the species became a giant species.
Sulcata tortoises are native to Africa inhabiting the northern grasslands and southern Sahara Desert. The proper scientific name is Geochelone Sulcata and designates Koopa as a giant land tortoise. Sulcata's are the third largest land tortoise species. He is a vegetarian and eats a high fiber low protein diet of grasses and occasional veggies like squash, pumpkin, and he loves carrots. I give him a carrot and refer to it as a carrot-kong (like the dog toy) he loves the color and ends up rolling it around as he gets little bites out of it. He'll let you pet him if you stroke him very gently, he lets me pet his little tortoise head which melts my heart. He doesn't prefer to be picked up, if you pick him up you really should let him stand on your hand so he has some "ground" under his feet or else he peeeeeeees all over you. He has small/medium size rocks as his substrate, many hiding places, a basking lamp and he gets sunshine every day. He loves when you give him a bath and rub him gently with a soft tooth brush, he even stretches out his neck so you can get his chin. He is my little nugget and I love him to death. This post was a brief, very very brief highlight of a few points in tortoise evolution and an introduction to my experience with tortoise ownership.