Evolution - Why Sea to Land?

[SkepticalScience]

How do we know that animals evolved from sea to land and not the other way around?

Why couldn't there have been some amphibious land creatures that decided to wander into the ocean one day. . . eventually leading to all the diversity we see in the ocean?

[Molinaro]

Because the oldest fossils found were from sea life. Older rocks don't have fossils.

However, there has been fossils found that show something that went from land, back into the water. Evolution doesn't preclude going the other way.

[Meffy]

Some land creatures did indeed wander into the ocean one day. Google for Basilosaurus. :-)

But that was LONG ages after the first living beings had left the ocean [edit: or tidal pools, etc.] where they originated and ventured onto hostile dry land. Simple single-celled creatures live in water, they can't cope with dry land. It took a long time to evolve to the point where living beings could exist in drier places.

[KingMerv00]

The oldest fossils were underwater at the time of their creation. Land fossils don't appear until much later.

From a chemical POV it makes more sense too. A liquid evironment allows many more reactions per second than an "open air" evironment. Water makes an excellent solvent.

[KingMerv00]

Just curious. What brought about this question?

[mummymonkey]

I've often wondered if early life was marine or freshwater? Amphibians today are almost exclusively freshwater animals yet many illustrations depict the first land creatures crawling out of the sea.

[Meffy]

I changed my post above to mention tidal pools. ISTR this is a popular idea nowadays -- evaporating pool of water shrinks, in desperation proto-scorp gulps horrid air... and discovers it can take it long enough to make it to the next, presumably less endangered, tidal pool.

[drkitten]

I've often wondered if early life was marine or freshwater? Amphibians today are almost exclusively freshwater animals yet many illustrations depict the first land creatures crawling out of the sea.

The various chemicals in modern animals blood serum and stuff strongly suggest that early life was marine. Similarly, many of the living analogues of ancient close relatives (e.g. coelecanths) are marine. Of course, that doesn't mean that there wasn't an intermediate freshwater step, but most of the smart money would bet on the ocean.

[Meffy]

When I was in elementary school we were shown a film that bored most of the other kits to tears but a few of us (not all the "smart" type either) liked it. "Hemo the Magnificent," the only reasonably long educational film I ever liked. Told all about the bloodstream. One of the factoids it presented was that mammal blood -- actually it said human but I'll be charitable toward the maker's bias toward his or her own species -- has, broken down to the level of individual chemical constituents, pretty much the same composition as sea water.

Some "lower" forms of life have open circulatory systems; that is, they're partially connected to the ocean water around them, not sealed off like ours. The more advanced along the phylogenetic tree we get, the more the circulation isolates itself from the surrounding environment. Without closed circulation an animal wouldn't be able to live out of water.

This is all memory from long ago. Anyone having better/more current info feel free to correct me.

[edit] Good heavens. I just googled "Hemo the Magnificent" and found who directed it. Frank Capra! =@.@= No wonder it was so outstanding.

[HeyLeroy]

Because there aren't any liquor stores in the ocean.

[canadarocks]

Some land creatures did indeed wander into the ocean one day. Google for Basilosaurus. :-)

Let's not forget the evolution of the whales, etc.!

[pchams]

Remember, that we still have to carry the water around with us.
Think amniotic fluid...

[Meffy]

Let's not forget the evolution of the whales, etc.!
Basil was their great-grandpa. :-)

[Apathia]

When I was in elementary school we were shown a film that bored most of the other kits to tears but a few of us (not all the "smart" type either) liked it. "Hemo the Magnificent," the only reasonably long educational film I ever liked. Told all about the bloodstream. One of the factoids it presented was that mammal blood -- actually it said human but I'll be charitable toward the maker's bias toward his or her own species -- has, broken down to the level of individual chemical constituents, pretty much the same composition as sea water.

Some "lower" forms of life have open circulatory systems; that is, they're partially connected to the ocean water around them, not sealed off like ours. The more advanced along the phylogenetic tree we get, the more the circulation isolates itself from the surrounding environment. Without closed circulation an animal wouldn't be able to live out of water.

This is all memory from long ago. Anyone having better/more current info feel free to correct me.

[edit] Good heavens. I just googled "Hemo the Magnificent" and found who directed it. Frank Capra! =@.@= No wonder it was so outstanding.


OMG! Hemo The Magnificent! A saw that several times coming up the system. The last time, the jerk of a substitute kept stopping to to make snide comments till we told him to shut up and show the film.

I bet there's some ID person out there asking why land life would go back to the sea. The assumption being that eveolution implies land life supeior.
But there's no superior life form in evolution except in the Theist version.

[Meffy]

Just so. Superior is a superior does... in a certain situation. There ain't no "this side up" sticker. :-) BTW, welcome and howdy.

[jimlintott]

Just so. Superior is a superior does... in a certain situation. There ain't no "this side up" sticker. :-) BTW, welcome and howdy.

As I always say, there is no big book that evolution gets to look up what to do next in.

[Silly Green Monkey]

I read recently that oxygen-using life started in the ocean because even when the atmosphere had very little oxygen, there was oxygen dissolved in the oceans.

[CFLarsen]

If those trying to eat you are all sea-bound, it makes VERY good sense to try to get to dry land.

[Meffy]

It's also your only chance at finally drying out your swimsuit. A bigger motivation than you might think.

[CFLarsen]

It's also your only chance at finally drying out your swimsuit. A bigger motivation than you might think.
Au contraire.

There is plenty of evidence that wet swimsuits increases your chances of passing on your genes...

[aggle-rithm]

Au contraire.

There is plenty of evidence that wet swimsuits increases your chances of passing on your genes...

...or it could immediately eliminate you from the gene pool, depending on how it fits you.

[aggle-rithm]

I was going to say that land animals carry water around with them, while marine animals do not carry air -- but that's wrong! Most fish have air bladders.

Interestingly, I saw something on television a while back that talked about fish with lungs, such as the lungfish (der!). These fish appear to be more primitive than most fish today. That's because most of today's fish evolved from lunged fish long ago... the lung slowly became the fish's air bladder.

Interesting, if true.

[CFLarsen]

...or it could immediately eliminate you from the gene pool, depending on how it fits you.
I think forum rules forbid me to link to...you know what.

[CapelDodger]

As I've heard the story, the first life on land was plant-life. There it escaped the constant depradations of animal-life, for a while. Animal-life, in the shape of insects, followed to exploit a whole new resource. Vertebrate life followed to exploit the whole new resource of the insects.

Land-life has returned to exploit the watery domain but has never, to my knowledge, reverted to water-breathing.

[CapelDodger]

I was going to say that land animals carry water around with them, while marine animals do not carry air -- but that's wrong! Most fish have air bladders.

Interestingly, I saw something on television a while back that talked about fish with lungs, such as the lungfish (der!). These fish appear to be more primitive than most fish today. That's because most of today's fish evolved from lunged fish long ago... the lung slowly became the fish's air bladder.

Interesting, if true.
Most fish have gas-bladders, not air-bladders. They've evolved to control buoyancy. Fish can make gas just as you and I can, and we did, after all, evolve from fish with gas-bladders. Fish today are far more sophisticated as fish than the fish that groped their way onto land a few hundred million years ago. Those fish were the marginalised, the losers of their time. The fish species of today descend from those that were able to compete in the big league. It's not surprising that they're more sophisticated.

[Tanstaafl]

I've often wondered if early life was marine or freshwater? Amphibians today are almost exclusively freshwater animals yet many illustrations depict the first land creatures crawling out of the sea.

From what I've heard the atmosphere a few billion years ago was very different from now, and I suspect the same was true of the oceans. I would imagine there was far less salt in them then, and only after several billion years of rivers dumping in with minerals, and water evaporating, did they get as salty as they are now. Not that I think any of this changes much of anything else that's been posted here so far though.

[athon]

From what I've heard the atmosphere a few billion years ago was very different from now, and I suspect the same was true of the oceans. I would imagine there was far less salt in them then, and only after several billion years of rivers dumping in with minerals, and water evaporating, did they get as salty as they are now. Not that I think any of this changes much of anything else that's been posted here so far though.

The balance of salt has actually been quite stable for the past few million years due to tectonic cycles, and should continue relatively so with small fluctuations resulting from changing currents and changing contributions of fresh water. There's no chance the oceans will one day become anything like the Dead Sea (not to say that wouldn't be fun - I love the Dead Sea).

Our blood sodium is roughly about 0.9% concentration, while present ocean salinity is about 1.05% sodium. The majority of oceanic sodium leached into the oceans from the seabed as they formed billions of years ago, with a small amount adding in from groundwater and rivers which has leached it from the land. In fact, various changes over the past few billion years mean that early life forms evolved in an environment that was 1 - 2 times saltier than today's oceans. Salt dissolved into early bodies of water very quickly it seemed.

So, early life forms were somewhat halophilic, and changed with time to tolerate lower salt concentrations. The fact that our own intracellular salt concentrations mirror a more modern ocean environment is a reflection of the relatively 'late' exiting of life from the oceans onto land.

Athon

[The Painter]

Fish today are far more sophisticated as fish than the fish that groped their way onto land a few hundred million years ago.

Yes, but the big question is; do they taste better?