Is it thought that hundreds of millions of years ago, there was no oxygen?

Question

If so, then how would early organisms have survived the UV light coming through without an ozone layer to protect them?

It is estimated that if no ozone was present today, organisms would be killed in 0.3 seconds! (from Tornado in a Junkyard)

Answer 1

Animals breathe oxygen. Only some types of one-celled organisms can live without oxygen.

Answer 2

Fennec ..you really need to read a broader range of evolutionary texts. Most of your questions are very easily answered with a minimal introspection of the peer reviewed literature. Creationist literature , such that it is , has no scieintific support and most of it perverts, distorts, or even fabricates data and data analyses. You seem to be interested in evolution if for no other reason that to try ( as the ID and creationists have tried ) to discredit or disprove it....well good luck with that. It is to biology what the periodic arrangement of elements is to chemistry. I guess the reason creationsits do not try to subvert that is that chemistry does not directly challenge their young earth..Biblical interpretation of "reality"

Answer 3

<<Is it thought that hundreds of millions of years ago, there was no oxygen?>>

No.  It's known that, until around 3.8 billion years ago, there was very little atmospheric oxygen, but the stuff then started to build up.  One indication of that process is the occurence of traces of rust, something that requires atmospheric oxygen.

<<If so, then how would early organisms have survived the UV light coming through without an ozone layer to protect them?>>

Living in places not exposed to UV light is one method.  Enough water provides protection as do layers of rock.  There are organisms living comfortably at depths in the ocean, and others living in rock layers more than a kilometre below the surface of the planet.

<<It is estimated that if no ozone was present today, organisms would be killed in 0.3 seconds! (from Tornado in a Junkyard)>>

I've no idea about the validity of that estimate.  However, it can't possibly apply to organisms living in places not liable to be exposed to UV light.  If whoever wrote 'Tornado in a Junkyard' didn't mention that detail, then they've obviously missed a significant (and stunningly obvious) point.  Given the title cited, that somehow doesn't surprise me all too much.

Update

<<So when animals first came out of the water, was there enough oxygen, and ozone, to protect them?>>

Yep.  That appears to have occurred within the last 450 million years.  By that time, there was plenty of atmospheric oxygen.  Tis often said that the forests are the lungs of the planet, seeing as plants produce oxygen as a by-product.  That ignores the detail that at least half of the oxygen supply comes from single-celled organisms in the ocean (phytoplankton).  So called phytoplankton went to work producing oxygen way before the development of any plants.

Answer 4

It is believed that the earliest orgnisms lived in the oceans, deep enough to protect them from UV.

The mass colonisation of the land appears to have happened only when atmopheric oxygen (from photosynthesis) was high enough to absorb UV.