We vomit if exposed to the smell of putrid flesh. Why? Because our ancestors were omnivorous and scavenging animals that often took advantage of carrion. Many animals, particularly large ones, rot from the inside out, so our forebears might easily eat some parts of an animal before releasing the gases produced by advanced internal decomposition. The presence of these gases would be a good indicator that the flesh eaten was itself contaminated, so our ancestors would be have been selected, if they had regurgitated it, thus reducing the probability of poisoning.