In ancient days, living things were believed to originate in two ways:
1-Biogenesis:
This was common observation that living things are produced from their parents.
2-Abiogenesis:
This hypothesis was formed due to lack of information. According to this concept living things are produced from non living things spontaneously. For example frog is produced from mud, mice from debris, insects from dew drops and maggots from putrefied meat.