It's worth remembering in these cases that the date of the earliest fossil find is only weakly related to the date of the earliest instance of the fossilized species: http://www.tjradcliffe.com/?p=1610
Sparsely sampled distributions will necessarily have tails that are not represented in the sampled data, and for early humans those tails could plausibly go back much, much further than the current record shows.
As such, we can expect that there will be a slow, steady stream of "surprising" early finds that push back hominid evolution to much longer ago than previously appreciated. The same is true of dinosaurs or any other species: the rarity of fossilization events is such that the probability of the earliest surviving fossil being particularly close to the emergence of the species is just not that high.
It would be interesting to do a serious analysis of the fossil discovery distribution to see if we can make a better estimate of the earliest emergence of hominids based on empirical estimates of fossilization and discovery probabilities.
How do you define an emergence of a species when these hominids are always in a state of constant change? Wouldn't this mean that there is no concrete way to determine if a fossil is from 'one side' or the 'beginning'(earliest point) of a species?
Well, this is an open question in evolutionary biology :)
One important thing to keep in mind is that species are hypotheses that can be tested, and the interpretations can vary from person to person and field to field. For example, many molecular geneticists will often discover new cryptic species on the basis of their DNA sequences, that a morphologist might not consider to be a true species. For fossils, of course, there almost always only morphological evidence for species.
The relative roles of anagenesis (change within a species) and cladogenesis (splitting event that creates two new species) in evolution is still being investigated, especially by paleontologists. One of many problems is that the fossil record is extremely incomplete, and so it's difficult to determine whether anagenesis or cladogenesis dominates. For me (as a non-paleontologist) my simplifying assumption about evolution is that anagenetic change doesn't occur, but again this is still an area of active research.
> It would be interesting to do a serious analysis of the fossil discovery distribution to see if we can make a better estimate of the earliest emergence of hominids based on empirical estimates of fossilization and discovery probabilities.
Surely someone's done this? It's reminiscent of the tank problem.
Sparsely sampled distributions will necessarily have tails that are not represented in the sampled data, and for early humans those tails could plausibly go back much, much further than the current record shows.
As such, we can expect that there will be a slow, steady stream of "surprising" early finds that push back hominid evolution to much longer ago than previously appreciated. The same is true of dinosaurs or any other species: the rarity of fossilization events is such that the probability of the earliest surviving fossil being particularly close to the emergence of the species is just not that high.
It would be interesting to do a serious analysis of the fossil discovery distribution to see if we can make a better estimate of the earliest emergence of hominids based on empirical estimates of fossilization and discovery probabilities.