Rather, early fish had lobed fins. Some of these fish evolved to have the ray fins that are most common. Some evolved into having legs. Some lobe-finned fish still exist, and some fish, like catfish, still use lobed fins to try to more effectively cross land.
None of this is easy to view as evolutionary progress. The most evolutionary biologists would call "progress" is simply noting that one descendent species is more different from an ancestor than a different species. But who is to say that an elephant is more progressed than a flounder?
There are two problem points where perhaps you wouldn't consider it mere splitting: at some point simple single-cellular organisms developed cellular systems, and at another point, these complex, single-celled organisms not only became multicellular (colonial), but a multicellular organism began to show cellular differentiation. That is, a colony of cells began to rely on specialized cells to reproduce, creating cells that were capable of starting a while new colony. At this point, colonies of single-celled organisms became multicellular organisms.
The second case isn't hard to figure. While it's true we haven'to bserved it happening, there are innumerable intermediate organisms.
The first one is a tougher case: Biologists call their solution, the endosymbiotic hypothesis. Two species of simple cells lived symbiotically so regularly, they became dependent on having each other as symbionts. Once that happened, an offspiring of the "outer" cell would not survive if it did not also have an offspiring of the "inner" cell within it. Given such a strong evolutionary force, it's not hard to imagine that the outer cell began to regulate the reproduction of the inner cell.
But how did the outer cell come to genetically control the inner cell (organelle)? We've seen that viruses and viroids transfer the location of genetic material. In fact, we've seen speciation occur on this basis. All that had to happen was that a virus transfer genetic material from outside a nucleus to inside it.
What if that never happened? Well, hold on to your hat: It so happens that that process never did happen in the instance of out mitochondria. Mitochondria are the organelles reponsible for turning carbohydrates into energy, the most vital organelle of a cell (besides the nucleus.) This is why they could use mitochondriato construct a family tree of the entire human race, and how they could figure out how many generations we are removed from our common grandmother! They have their own DNA, called mDNA. Mitochondria evolved entirely seperately the rest of our cells, and do not participate in sexual reproduction. Each mitochondria is a clone of its own parent.
These systems have self-replicating mechanisms, self-healing mechanisms, reproductive mechanisms, fuel refining digestive mechanisms, self-cleaning mechanisms, highly refined oxygen filtering mechanisms, waste disposal mechanisms, cooling mechanisms, heating mechanisms, nervous systems, highly sophisticated coverings over structures that attach the energy transfer systems together.
Consider a CPU that can drive a car 60 mph on a winding cliff faced road, while singing a melody, while discerning the wind blowing over every exposed hair, while regulating the motion of a 2000 pound moving object using precise predictive visual stimuli, while cooperatively leaning every muscle of the body into a turn, while recognizing the pitch of the road to calculate the minute steering and braking adjustments, while responding to memories of how the suspension will react at precise angles of momentum, while enjoying the millions of hues the sunlight reflecting produces on a multifaceted ocean.
The production of one of these systems has not been shown to be true. In fact it goes against every common sensical concept of chemistry, biology, physics and mathematical information theory.