1) Seed-dispersal caused amphibians by feeding them.
Before amphibians evolved about 370-mya (million years ago), seed dispersal was a huge problem for plants. Wind and insects are relatively effective for pollination, but not for spreading seeds. So before amphibians evolved, most plants could only expand their territory by a few meters a year.
At this time, water-born seed dispersal would spread seeds farthest, and this caused nature to favor the plants that would disperse their seeds via waterways. This was also the age of cycads, or palm tree like plants, and many cycads still spread their seeds via water, for example the spouted coconut and atap seeds below.
Fish evolved to feed on the nutritious water-borne seeds and spores, which at first had no protection from animal consumption. And little doubt, the struggle for nutrition drove some fish to begin entering shallower and shallower waters, and then finally on dry land to get more of the nutritious seeds.
Now rains would occasionally wash the seeds of nearby plants into the waterways, so many plants that were merely near rivers (and small tributaries) were also being favored by river-based seed dispersal. These probably evolved to drop their seeds throughout the year because heavy rain would carry would often carry their seeds away. Therefore we imagine that the areas around the world’s rivers and brooks were full of nutritious seeds until the arrival of the proto-amphibian fish.
At some point, a remarkable thing happened. Some undigested seeds began passing through this terrestrial fish and onto land, some distance along the river, and far from where the seeds came from. Once this seed-dispersal symbiosis evolved, the plants participating in it had an advantage (a seed and territory spreading advantage) over all the other plant species. And at the same time, the “land-fish” eating the seeds of these plants were spreading the territory of their symbiots, so their food supply was constantly being enlarged by their own feeding.
Here is an Axolotl, a salamander with external respiratory organs. This animal shows us how walking on land evolved before the ability to breathe air.
Thus the initial limiting factor seems to have been the ability of the fish to breathe air and move far from the water in search of seeds. And the individual fish that could do this was both best at finding food and dispersing the seeds of its symbiots. Thus the benefits of seed spreading favored the fish that could venture farther and farther onto the land to find seeds to eat and spread. And this is the evolutionary hurdle that caused the amphibians, beginning about 370-mya. This is also where the conifers (pine, juniper, holly), apparently came from, also about 370-mya. The timing coincidence is because the two were apparently symbiots.
2) Seed-spreading caused reptiles by feeding them. Seed spreading pulled vertebrate life far from the water by feeding it.
About 320 to 310-mya, we start to see animals that did not go back to the water to breed, namely the reptiles. This was about the same time when cycads evolved. Cycads are palm-like plants which today put coconuts and giant bundles of dates high up a long stalk. The early cycads were however short.
Some experts think the first proto-cycads began around 325-mya, although some experts with a stricter definition of a cycad think that it was as late as 280-mya. Thus due to timing, it appears that the fruiting cycads were the main symbiot of the first non-amphibious reptiles. Thus it appears that seed dispersal also pulled animal life away from the water and caused the reptiles.
3) Seed spreading caused plants to offer fruit
After some time, the plants began helping their symbiots (and the dispersal of their own seeds) by providing fruit nutrition outside the seed itself. This is why we have fruits of two parts. One part is tasty, soft, sweet fruit, and the other is the hard inedible seed, like with an avocado, peach or mango. The delicious edible fruit evolved as the symbiot-helping bribe. The hard inedible seed evolved as the part to pass through the digestive tract of the symbiot.
This fruit bribery is what the angiosperm plants (the flowering and fruiting plants) evolved for. And we do find fossilized angiosperm-like pollen from about 245-mya. And while the first dinosaur fossils officially come from about 225-mya, their proto-dinosaur ancestors could have easily been angiosperm symbiots say 20-million years earlier.
Here is a durian sliced in half. Note the thick, woody, and spine-encrusted husk. Note the big seeds within a thick layer of fruit. If this durian has 3-kilos of fruit, it will have 4,500-calories of nutrition.
The arms race of the plants (to spread their seeds farthest) led them to give all they could to their most beneficial seed spreading animal symbiots (as we see in the hugely nutritious durian above). Thus the benefit of terrestrial seed spreading caused the plants to draw the fish out of the water by nourishing them. Then seed spreading pulled the tetrapods (4-legged animals) far from the water. Then, in a similar way, seed spreading caused the plants to draw the animals into the treetops for reasons that we will get to next.
4) Seed-spreading caused dinosaurs to be big
5) Seed-spreading caused tall trees with macro-fruit
Lets go back to before birds and bird-based seed-spreading evolved. At this time, if plants wanted animals to spread their seeds, they had to give away a fruit-snack just like today. But at that time (before birds), the seeds swallowed by the biggest and highest-reaching animals tended to be spread further, giving that tree species an important advantage over seeds swallowed by smaller creatures that did not need to constantly roam in search of food. So before the evolution of birds as effective seed spreaders, survival favored trees with giant, nutritious fruits high up, along with their giant animal symbiots capable of reaching this fruit. This is the evolutionary force that both caused dinosaur gigantism, as well as tall trees.
Now saying that dinosaurs also ate fruit is a bit of a heresy for paleontologists. This is because there are many dinosaur coprolites (fossilized feces) that have been analyzed, and none show any evidence of sweet fruit in the dinosaur diet. They show evidence of leaves and pine cones, but not sweet fruit.
Coprolites are however quite rare, and they are even rarer inside dinosaurs. Also, we don’t find any large fossilized sweet fruit outside of dinosaurs because high-energy fruit, like high-energy dinosaur soft tissue normally breaks down completely, unlike low-energy pine cones and leaves. So it seems to be that the presence of fruit in the dinosaur’s gut causes all the gut contents to break down completely, and we only find coprolites when dinosaurs were eating low-energy foods.
6) Seed-spreading caused Dangling fruit on break-away stems
After some time, small energy efficient animals evolved to climb the trees to steal the macro-fruit (durians, jackfruit etc.) and cheat the dinosaur (mostly sauropod) seed-spreading system. This caused the fruiting plants plants to evolve dangling fruit on break-away stems. At first this boobytrap, or counter-measure must have been highly effective at preventing small animals from cheating the system.
7) Seed-spreading caused birds
Today the main theory of the origin of the birds is that they evolved jumping down on prey animals. This theory however works better if we use fruit instead of prey animals. Thus the birds came from the cheating animals that survived the fall when the dangling fruit broke-away. First they survived small falls, then larger falls, then falls from any tree. Then they evolved to conserve energy jumping and gliding between trees… of greater and greater distance. Then they were sort of flying.
8) Seed-spreading caused mammals
The mammals came from the animals that were so small they either didn’t trigger the fruit to fall, or so inconsequential their group would prosper anyway if they died bringing down a giant jackfruit that would feed their entire group for a long time.
9) Seed-spreading ended the giant dinosaurs
Once birds evolved sufficiently, plants had a more efficient means of spreading their seeds. At this point, the plants that continued to provided macro-fruit for dinosaurs were wasting huge amounts of energy on seed spreading. These were at a huge disadvantage and eventually these plant species either adapted or mostly died out. Then once the macro-fruit trees all started catering to the birds and their tiny appetites, the dinosaurs had less and less food to eat. Ultimately a particularly large climate shock delivered the final blow.
An integrated theory of tetrapod evolution
Here is a single unified theory that explains most of the most important developments in land animal evolution.