The first obstacle animals had to overcome so that they could live on land was the pull of gravity. Many fish that became marooned on dry land died off fast, due to the force of gravity on their gills. To overcome the problem, some animals evolved and developed a body cavity filled with fluid that helps strengthen the body. Other animals, such as vertebrates evolved and developed a skeleton to effectively hold up their bodies against earth's gravity pull. .
In addition to skeletons, many of those animals also evolved and developed limbs so that their weight can be supported. The advent of flexible joints in the limbs made these animals capable of movement.
The next obstacle animals had to overcome was water loss. Water loss is dangerously fatal and as a result, animals evolved and developed watertight/ waterproof coverings to reduce evaporation.
The last obstacle animals had to overcome was reproduction on land. Animals adapted themselves to reproduction far from water with the advent of internal fertilization, that way the sperm won't dry up on their way to the egg. Birds and reptiles lay eggs with hard shells and chorion impermeable to water.
Humans withstand gravity via our internal skeleton. Human legs and the bones that support them, are strong as they support the weight of a person against gravity in addition to providing movement. Human skin serves to protect us from the elements, and for life on land away from the water.
Humans reproduce on land by way of internal fertilization, so the male and female gametes do not dry out. The embryo develops inside the female, therefore eliminating any chances for predators to eat the young.
