As far as I'm aware, none really (yet) knows EXACTLY "how it all started", whether the "it" is the beginning of life or the beginning of the Universe itself. (The creation of the Universe, and the unknowns surrounding that, are frequently cited in a similar manner to abiogenesis as requiring us to conclude that a conscious Creator is responsible.)
There was a time when mankind could explain almost nothing of how the world we see came about. At that time the choices were to attribute creation to a creator or to just shrug and admit ignorance. Inexplicable events (such as lightning and rainbows) were therefore attributed to supernatural/religious causes. Since then we've come a long way in understanding the world around us. Most of us no longer attribute lighting or rainbows as divine events because they are understandable consequences of purely physical phenomena. There's little doubt that our ability to understand the world in terms of physical causes and effects has reduced the need to invoke the divine as an explanation of the things around us.
There are still things we can't fully explain. Depending on whether our pre-existing beliefs are religious or 'sciency-atheist', we tend to take one of two approaches to things that are currently (far) beyond science to explain. The religious are likely to believe that since a science-based explanation seems impossible the 'cause' must be a god. The sciency-atheist will tend to believe that we just can't explain these things 'yet' but that continued investigation will provide an explanation.
The religious view, of course, has a 'get out of jail free' card, in that it can claim that the invoked (non-denominational) god requires no explanation of origin - so the sciency-atheist may think that the religious 'explanation' is no explanation at all, and effectively amounts to a shrugged 'dunno'. Conversely, the religious may accuse the sciency-atheist of taking an unscientific leap of 'faith, since even where current scientific thought might suggest that an event (e.g. abiogenesis, or the Big Bang) is so unlikely as to be virtually impossible or inexplicable, the sciency-atheist will tend to believe that given sufficient time, thought and research we will eventually understand it and realise it's a purely physical phenomenon.
One upshot of all this is that unless or until we can explain absolutely everything scientifically (and as we're a LONG way from that), the religious will tend to believe that the scientifically inexplicable is evidence for a god. So the sciency-atheists will (virtually) never win over the determinedly religious with science. Conversely, the sciency-atheist will always believe that the currently inexplicable will one day be explained by science (or at least that understanding of anything that cannot be explained scientifically is not clarified by invoking divinity) so the religious will never win over the determinedly sciency-atheist by arguing the necessity of divinity to explain the observed world.
