Any autonomous machines in a distant enough future will look neither as the present humans or animals nor like the present robots, because they will have to use a mixture of the technologies developed by living beings and by humans.
Humans have developed various things that are better for certain purposes than anything used by a living being, e.g. metals and semiconductor devices (most of the human-only technologies are a consequence of the control of fire, i.e. of the ability to perform manufacturing processes at high temperatures, unlike the living beings, which are limited to temperatures close to that of the ambient). On the other hand, for other purposes the use of organic substances and of the methods of chemical synthesis used by living beings are unbeatable.
So any future descendants will have to use hybrid technologies, like the "cyborgs" of many SF novels/movies, except that I have never seen any SF "cyborg" that combined the right parts from "machines" and from living organisms.
As an example, for the problem of energy storage for powering a machine, for providing short-time high power bursts capacitors and batteries are better than the chemical reactions used by living beings, e.g. ATP & phospho-creatine hydrolysis and anaerobic glycolysis. On the other hand for storing high-amounts of energy for long-time autonomy at a moderate power level, none of the fuel cell or battery technologies that have ever been attempted appears to have the potential to ever match the performances of the enzymatic oxidation of hydrocarbons that converts their chemical energy into ionic gradients in living beings, e.g. in our mitochondria. So an ideal autonomous machine would combine high-power capacitors/batteries with high-energy biologically-derived fuel cells.