Again, this is a gross misunderstanding of the nature of science, which is constantly building upon the ideas of the past. Science does not just change, it builds on the past and goes beyond to the future. It does make mistakes aplenty, but the self-correcting feature of the scientific method is one of its most beautiful assets. Hoaxes like Piltdown Man and Calaveras Man, and honest mistakes like Nebraska Man and Hesperopithecus, are, in time, exposed. Science picks itself up, shakes itself off, and moves on.
All causes have effects.
That is, the cause of intelligence must be intelligent. Also, regress all causes in time and you must conclude that there was a first cause - God. Animal manure makes fruit trees grow better. Purpose is, in part, what we are accustomed to perceiving. Finally, not everything is so purposeful and beautifully designed.