Complex sentence: An alternative theory states that humans hunted the elephant birds to extinction in a very short time for such a large landmass (the blitzkrieg hypothesis) or is the possible secondary effect of human impact by possible transfer of hyperdiseases from human commensals such as chickens and guineafowl.
Split into simpler sentences: An alternative theory states that humans hunted the elephant birds to extinction in a very short time for such a large landmass (the blitzkrieg hypothesis). It has also been suggested that the extinction was a secondary effect of human impact due to transfer of hyperdiseases from human commensals such as chickens and guineafowl.