Those who come to the human form through the quality of tamo guna were monkeys in their last life.
By fulfilling the process of evolution from the aquatics to the animal platform, a living entity eventually reaches the human form. The three modes of material nature are always working in the evolutionary process. Those who come to the human form through the quality of sattva-guna were cows in their last animal incarnation. Those who come to the human form through the quality of rajo-guna were lions in their last animal incarnation. And those who come to the human form through the quality of tamo-guna were monkeys in their last animal incarnation. In this age, those who come through the monkey species are considered by modern anthropologists like Darwin to be descendants of monkeys. We receive information herein that those who are simply interested in sex are actually no better than monkeys. Monkeys are very expert in sexual enjoyment, and sometimes sex glands are taken from monkeys and placed in the human body so that a human being can enjoy sex in old age. In this way modern civilization has advanced. Many monkeys in India were caught and sent to Europe so that their sex glands could serve as replacements for those of old people. Those who actually descend from the monkeys are interested in expanding their aristocratic families through sex. In the Vedas there are also certain ceremonies especially meant for sexual improvement and promotion to higher planetary systems, where the demigods are enjoying sex. The demigods are also very much inclined toward sex because that is the basic principle of material enjoyment.
Source: A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “Srimad Bhagavatam”, Fifth Canto, Chapter 14 – Text 30
