So IYO speciation or naming different species is a "sampling" technique of the greater analog evolution process. So Neanderthal is a sample of a greater being with traits similar to modern humans. I will consider that but I still see it as step function with species suddenly "appearing", paleo biologists use this phrase all the time.
Sure, naming different species of fossils is absolutely a "sampling technique", since we only have a very small percent of all the pre-historic fossil types.
But let me also mention that we have 100% of all living breeds, sub-species, species, genera, families, etc.
And scientists have made considerable efforts to define criteria for putting each known critter into its appropriate category.
If you just consider mammals, for example, today there are over 5,000 species.
Now consider that the average mammal species survives around one million years before going extinct (or evolving into something else).
So, if we go back in time, let's say 10 million years and ask: how many fossil species of mammals do we have from 10 million years ago?
The answer is probably a few dozen, meaning circa 1% of all the species then living were fossilized and now recovered by science.
As for how many closely related pre-humans there were, besides Neanderthals, in recent years four more have been discovered -- the so-called "Hobbit" from Indonesia, Denisovans from Siberia, Deer Cave People from China and Idaltu's from Ethiopia.
Those are the ones we know about, but presumably are just a small percentage of the total number of near-human who may have lived until fairly recent times.
central_va: "I will consider that but I still see it as step function with species suddenly "appearing", paleo biologists use this phrase all the time."
Of course they "suddenly appear" in the fossil record, but that does not suggest they "suddenly appeared" in nature.
As we can see among living groups today, a totally new species does not happen overnight, but rather in small steps, each step making the new group slightly more distinct from its parent population.
Eventually they no longer routinely interbreed, and so scientists re-classify them as a new species.