waht said:
Every single evidence coming from cosmology, astrophysics to evolutionary biology points overwhelmingly to the fact that all the stars, elements, compounds, and life were created in a bottom-up way, where more complex building blocks arose from simpler building blocks coming together.
Science also has room for top-down causality - global constraints. And also for theories of meaning - semiotics being an example.
If you study systems science, hierarchy theory, ecology, neuroscience and other stories of self-organisation, you can see that bottom-up causality - local atomistic/mechanical construction - is only half the total story.
Science has become publicly identified with bottom-up causality of course. Indeed, it is what most computer scientists and physicists also
believe 
. Which has left an unfortunate marketing gap for theists to claim ownership of top-down causality.
Though as is remarked, theists can also claim the whole bottom-up science story is set in train by a wind it up/let it go style god.
It is interesting to ask what you will end up believing as the meaning of life if instead you are an atheistic systems scientist.
It used to be that a lot of holists and systems scientists did believe that some spiritual-like comfort and meaning would lie in this "whole of things" approach. It attracted a lot of catholic scientists and new agers as a result.
Then growing out of this has been a more techno-triumphalist theory of meaningful existence. Kurzweil's singularity, Tippler's Omega point and de Chardin's noosphere. The idea here is that complexity is the magic word. The natural goal of the universe is to become as full of life and mind as possible. Humans define what is good (as we are the most sophisticated level of intelligence) and if we can improve on ourselves, spread our influence everywhere, then this would be achieving a natural purpose.
Man becomes god, in effect - an omnipotent and omniscient presence in the universe. And this is also linked to a reversion to a more bottom-up view of complexity - the Santa Fe brand where top-down causality is again "just emergent" and so not terribly "real". The techno-triumphalist view is based on exponential progress that recognises no necessary upper limits, no global system constraints.
All very science fiction. In fact why not call it a scientific religion? Most scientists are really working as technologists and this is a creedo that endorses their actions. They are busy building that better world and to the extent this is natural, a purpose embedded in reality, it gives individual lives meaning to be an active participant in the process.
This is what gives their rhetoric a righteous tone (one to match other camps of religion). The purpose of science is to transform energy and resources into creating a vaster, more all-knowing, all-controlling, version of the human mind.
So you have at least three choices when it comes to finding the meaning of life. And scientist or theist, you can slot into any of these three I would say, even if some choices are more favoured.
1) bottom-up causality - claims simple and meaningless beginnings. Meaning is something that you then have to construct. But there are no limits, so what is natural is to head for infinite construction. The singularity for instance.
2) top-down causality - claims that something global, larger than ourselves, is in charge and enshrines purpose, meaning, goals. Which to a theist sounds like god. To attain a meaningful life, we have to rise up somehow to share that level of existence. There are also global limits as we should be entrained to that global purpose. We are not free to construct our own human meanings. And one 'obvious' way to move up to a higher level of meaning is to de-construct our material existence - to be poorer, humbler, less individually assertive and active, etc. By giving up our bottom-up approach, we become more purely aligned with the greater reality of the top-down, more purely aligned with where the meaning of reality exists.
3) a true systems view stresses that reality is self-organising and is the result of an equilbrium balance between bottom-up and top-down causality. So neither is privileged and meaning exists in the balance. A meaningful life is one that balances personal action against wider (typically social and ecological) constraints. Both individual competition and general co-operation are valued evenly, in systematic fashion.
What kind of "religion" might arise out of (3)?
Here there is a problem as there are two kinds of equilibrium systems - the open and the closed, the dynamic and the static. Do you "worship" the second law, the dissipation of entropy gradients, and so look forward to the heat death of the universe (and hence want to do all you can to accelerate this fate?). Or do you instead argue that the stable persistence of a regime is the key, and so a meaningful life is one that helps the human social system from crashing off the road, staying within its given ecological constraints?
Anyway, my points here are first, that it is too simple to identify science with bottom-up causality and religion with top-down (though there is some truth to that).
And second that science also seems strikingly like religion once people start talking about their futuristic (to be constructed!) idea of "heaven" - as in a singularity scenario, or a global consumer paradise, and other techno-utopias.
And third, if you progress to a systems perspective of the possible meaning of life, the answers could be surprising (worshipping the second law and the heat death?). But also still not easy to settle. A fundamental polarity still exists between maximising complexity and maximising simplicity - between keeping the human game going as long as humanly possible and crashing and burning in the name of accelerating the universe's heat death.
(You get the point)