What situation allowed for evolution to move creatures from water to land?

Why does life spread to every available niche? To better survive Natural Selection. Every organism must gather resources to survive and reproduce. Different adaptations allow different organisms to use the resources in different environments. Adaptations that permit the use of new resources or gives better protection from predators would be advantageous and increases the odds of reproducing and passing down these genes.

How did it happen? Like any other development. Slowly, over many generations.

Imagine an organism living in a old, established niche. It's crowded with many others and competition for limited resources is intense and dangerous predators lurk everywhere. Just outside is a wide open, unclaimed space with abundant resources and a refuge where the old predators can't go. The organisms that had traits that permitted them to better utilize the new environment would have a significant survival advantage.

Not perfect, just better than the competition. These traits vary in a population, even in the same species. The best collection of traits confers an advantage to that individual which is more likely to have offspring and pass down the genes for these traits. Subsequent variation in subsequent generations plus Natural Selection pushes the development of advantageous variations. It could be something as small as a mutation that alters the growth pattern of the bones in fins. That mutation for the slight advantage was passed on and new mutations built upon earlier ones eventually led to fish that could move better out of water. Legs didn't just pop up. Limbs were built out of what was available to work with. You can trace the pattern of bones in fins and match them to bones in arms and hands.

It also went the other way too. After a while, the land becomes crowded with it's own food chain. Some mammals have taken the path back to the oceans to try their luck where our ancestors left hundreds of millions of years ago. We can see the intermediate stage of this in the Hippo. Hippos are the nearest land relative of whales and dolphins. Even though they live on land, they have many traits that permit them to use aquatic environments. They lack hair, secrete oil and communicate through underwater vocalization.

Closely related to both whales and hippos are cows. Slight changes over many generations led from cows to hippos to whales. Likewise, many generations led from fish to land using fish to amphibians to true land dwellers.

As for lung development, it takes a series of incremental steps from the gas-filled swim bladder to change it into a respiratory organ. Gills were maintained as long as the ancestor fish still lived in water so the swim bladder could mutate without affecting breathing underwater. Steps include partitioning and branching of the swim bladder to increase surface area for gas exchange. Also there were changes in the bladder lining to reduce water loss in breathing.

Again, it doesn't have to be a fully functioning lung, just a better one than the competition. If one fish had a bladder with a little more surface area for gas exchange, it could stay out of water longer, so it could get resources beyond the reach of the competition and stay out of reach from sea predators for longer. Changes in bladder lining happened in small steps too such as mutations in the thickness that improved gas exchange, or secretion of a mutant lipid to line the "lung" to provide better elasticity to maintain alveoli. You can see the incremental changes by looking at the lungs of other animals. The reptilian lung is not as well branched or clustered with alveoli and lacks lipofibroblasts compared to mammals. Amphibian lungs are simpler still with fewer alveoli and branches. It retains more of the myofibroblasts, originally used to pump the swim bladder, and they have been lost in reptiles and mammals. Land animals, including humans, go through a phase as embryos where they have gills, another reminder of the legacy of our ancestors.

Our lungs are not the only lungs. Our eyes are not the only eyes. Our ears are not the only ears. Earlier forms of these organs are found in other animals where they have been maintained because they are good enough for the environment of the organism. Having a more complex organ is offset by the cost of making and maintaining complex structures.

Evolution From Water To Land

One of the most important milestones in the evolution of life began some 400 million years ago, when the first animals made their way from water onto land. At that time, known as the Devonian period, the world was changing dramatically: complex plant ecosystems formed on land, the first woody plants appeared, and the water's edge was becoming a new kind of environment.

The move to land was a very gradual process, and the evolution of limbs wasn't a simple adaptation resulting from animals crawling onto the shore and never looking back. In fact, the new picture of this transition shows that most of the changes needed for life on dry land happened in creatures that were still living in the water. Some fishlike vertebrates had already begun to evolve limbs by around 400 million years ago: They were called "lobe-fins," with fins that looked like fleshy paddles, and they had lungs as well as gills.

It wasnt a case of "needing to go". Think of any population. some will go! once a population is using land as a means for say food source or protection from predators - some part of that population will adapt towards it.

Do not think of it as a fish flopping onto the land and saying "I can handle this". More like a wave of arthropods handle it first, amphibians are living on the water/land, some spend more time on land than not. think hundreds of millions of years to adapt.

It wasn't one situation but rather a large number of small events.

Fish have an air bladder to help bouyancy. When in shallow water, say to escape a predator, to find food (algae) or to reproduce in safety, there is less oxygen. Having the air bladder involved in gas exchange provided an advantage - the fish could go into shallower water.

Fish have fins. Stiffen them and moving in shallow water is easier and faster. Again, this provides an advantage when trying to escape predators, finding food, or a mate!

Fish have scales. Make them smaller, like the size of a cell, and add layers and you have skin. Add a protein, keratin, and the skin is water-proof.

There are other adaptation needed, eyes, digestive tract modifications, etc. and the process is slow. The motives, though, are simple...survival.

Darwin's survival of the fittest is not the strongest but the one who reproduces the most. If there is more food on land, than being on land is an advantage. If there are less predators on the land, than being on land is an advantage. And if your mate is on land...that's where you want to be!

Well 1st land life evolved as those that could live both water and land....mostly because the oceans are very undernourished. Land has much more in way of food so it was a vast advantage to be able to go on dry land and get it, also to be able to move from one body of water to another as needed.

Towelie> ah and again I see we disagree again. micro evolution IS macro evolution..just in the short term....but you think whatever you want, doesn't affect me..being right helps me sleep at night, but that's my thing I guess

1. Many more individuals are born than can possibly survive, thus there is competition for limited resources

2. Within this vast number there is variation, and because of this variation some of these individuals will have an advantage--however slight--over others

3. The ones who have the advantages are more competitive and thus they are more likely to obtain the limited resources

4. The ones who are succeeding in securing the limited resources are more likely to reproduce and thus pass onto their offspring the more competitive traits

Darwin

in this case ur talking about the limited resource of territory....

GL

The development of the chitinous exoskeleton. In water, it provided support and armor for the arthropods. On land, it additionally provided protection against desiccation.

There was more CO2 and sunlight on land, so plants moved to land. Animals moved to land for increased oxygen created by the plants, more food (plants), and to escape from predators.

maybe food supplies wheren't enough in the water so they started reaching the shore.maybe their oxygen capacity was big so they started moving to land slowly. maybe they were bored of swimming..maybe they thought Ariel the Murmaid was a true story.