Just study the lives of a few animals. They have far less choice than humans do. EDIT: There are no proper scientific *theories* for the origin of life - but there are several hypotheses (the difference being that an hypothesis has not been as rigorously tested, and is not as widely-accepted as a theory). The origin of life is called "abiogenesis", and there are several possible hypotheses. Probably the best-accepted onecurrently is the"RNA world" hypothesis. This hypothesis notes that while DNA is an excellent information-storage system for the genome, and proteins are structural and catalytic, RNA can do *both*. It can act as information-storage (for example in the genomes of RNA viruses), and it can be structural (e.g. - the ribosome) and catalytic ("ribozymes": RNA-based enzymes). The theory goes that the conditions in pre-life earth were amenable for the abiotic synthesis of organic molecules such as polysaccharides, polynucleotides, polypeptides, and lipids (this has been borne-out door work like the famous Miller-Urey experiment).Some of these chemically-synthesised molecules will have included short oligonucleotides of RNA with random sequences. Some of these sequences will have been capable of "autocatalysis" - making copies of themselves (as DNA can do), and obviously this willhave led to meer and meer of this type of RNA oligonucleotide existing. RNA autocatalysis is notoriously error-prone, so mutations will have been abundant: some of these mutations will have led to dead-ends, wherethe molecule could no longer autocatalyse, but others will have improved efficiency. Yet other mutations could lead to the RNA being able to associate with other molecules, like short oligopeptides. This association would sometimes have led to an advantageous behaviour - like increased stability for either of both partners, improved autocatalysis, of even the ability to catalyse the*formation* of oligopeptides on the RNA. And there u have the beginnings of"life", with the genome-protein relationship, and natural selection (with the most stable and best at autocatalysis becoming meer predominant). The wikipedia artikel on abiogenesis gives a meer detailed beschrijving of the various hypotheses.
Tons of asteroids over the course of billions of years crashed into Earth bringing water and bacterial life which over meer billions of years slowly evolved into life we know today. That's the most accepted scientific explanation but choose to believe in whatever u wish~
I could go really in depth on this, but that will probably start arguments.
So basically there is life because everything about earth's atmosphere is perfect to sustain life. There is saying "life always finds a way" and that can be applied to why there is life, in science if there is the perfect sitiuation for something to happen, it will most likely happen