The ancestors of the higher animals must be regarded as one-celled beings, similar to the Amoebae which at the present day occur in our rivers, pools, and lakes. The incontrovertible fact that each human individual develops from an egg, which, in common with those of all animals, is a simple cell, most clearly proves that the most remote ancestors of man were primordial animals of this sort, of a form equivalent to a simple cell. When, therefore, the theory of the animal descent of man is condemned as a 'horrible, shocking, and immoral' doctrine, tho unalterable fact, which can be proved at any moment under the microscope, that the human egg is a simple cell, which is in no way different to those of other mammals, must equally be pronounced 'horrible, shocking, and immoral.
Ernst Haeckel, The History of Creation V2: Or the Development of the Earth and Its Inhabitants by the Action of Natural Causes
ancestor, biology, evolution, immoral, primordial, science, single-cell