It is, exactly speaking, the legal provisions, which are found in the Code Digesta promulgated by Justinianus in 533. The code consists of the collected famous Roman lawyer's opinions as arranged in a form of legal provision. Three provisions said to be written by him may be interesting to us. Natural law treats all human beings as equal. According to natural law, all human beings are originally born free.
Natural law is the law which nature taught all animals.
The perspective here is limited to human beings. The message that not only human beings, but flying birds, fishes swimming in the water, animals were naturally, originally common, however, stimulates me again. Because, if it is the contemporary message that all living things including human beings owe their life maintenance and development to the fundamental, DNA we may perhaps feel the new message somewhat interrelated with the old one from the ancient Rome. The study of DNA, gene is intimately related with topic of evolution. That is why modern thinkers tended to criticize such a contradiction or discrepancy.