Could you go to the beach and enter the water safely billions of years ago here on earth? In the sense that there used to be different species of marine animals and fish that could be dangerous to humans.

Just going back 1 billion yeas will put you in the Protozoic Era. It would be pretty safe for humans. There were harmless sponges and single cell animals at that time. No fish, reptiles or marine mammals.

Just don’t drink the water untreated.

Just as today, it would almost certainly depend on WHICH beach you had in mind.

@gondwanalon I wouldn’t be too eager to put my trust in the benign nature of those single celled protoza. Our lungs and bloodstreams resemble to a great extent those bygone tropical seas in which we evolved. And then there’s the question of whether the atmosphere of the period would be breathable.

@HP – good point! I think it wasn’t – too much CO2, not enough O2.

@HP the atmospheric differences a billion years ago and today goes without saving.

Also there’s an interesting theory that the ancient seas had potassium (K) in high concentrations and sodium Na in low concentrations. This is why vertebrates today have cells intracellular fluid with high concentrations K and low concentrations of Na. Later the seas slowly became high in Na and low K. Animals adapted to the new sea by evolving a Na K pump (in their cell membranes) that pumps K into the cell and Na out of the cell. This keeps intracellular fluid high in K and Low in Na (the same concentrations as the ancient sea where it evolved from). At the same time terrestrial vertebrates carry with them the modern sea (vascular fluid or blood) which is very close in tonicity to the seas today.

In other words when vertebrates left the sea they actually carried with them two seas. The ancient sea (intercellular fluid) and modern sea (vascular fluid) and to keep the two seas separated we evolved the Na+K+ pump in our cellular membranes.