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This is for AP Biology, but the assigned textbook chapter doesn't contain any information about the question.

What are some difficulties animals had to overcome to reproduce on land?

Some disadvantages I brainstormed are

  • the formation of bones—need for vertical structure and support against gravity (?)
  • development of lungs

Are there any more?

Thanks,

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    $\begingroup$ Are you asking about specifically reproduction or land-survival in general? $\endgroup$ – WYSIWYG Nov 19 '14 at 9:35
  • $\begingroup$ Are you thinking of vertebrates specifically? Or animals more generally (including insects, mollusks, etc.)? $\endgroup$ – kmm Nov 19 '14 at 22:44
  • $\begingroup$ Just reproduction. Hm, no really specifics—any animals. $\endgroup$ – Sentient Nov 20 '14 at 5:42
  • $\begingroup$ I would have thought finding a way to get away from spraying a bunch of eggs on the water bottom, and then spraying them with a bunch of sperm and hoping for the best would have been the initial problem. Sea creatures that carry the developed eggs around would have been one approach (sea horse does that?) Impregnating females to carry around eggs would have another approach (reptiles, then snakes on land for example). Eventually leading to live birth, and the mammals that returned to the sea? (dolphins, whales). $\endgroup$ – CrossRoads Dec 3 '18 at 18:57
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The disadvantages you mention have not much to do with reproduction, but with locomotion and oxygen absorption.

Difficulties to reproduce on land are the fact that aquatic egg-producing creatures typically lay soft eggs with porous shells to promote exchange of water and waste products with the surrounding water. Water is essential for growth as a typical creature consists for some 70% of water. Soft-shelled porous eggs when laid on on land dry out very quickly and are prone to predation.

The main adaptation to reproduce on land instead of water has been the development of the amnion. The amnion is basically a membrane filled with water (the amniotic sac) in which the embryo is immersed during development. Hence, you could argue that the transition to land in terms of embryonic development has never taken place, as the embryos develop in a protected aqueous environment contained within the amnion! The early amniotes laid eggs on land where the embryos develop, while the later ones such as humans carry their embryos/fetuses around. But early as well as late amniotes are characterized by a developing embryo immersed in a watery environment contained in the amnion. Other structures that aided the transition to land were eggs with hard shells, and the allantois. A hard outer shell in amniotes that lay their eggs on land helps to protect them against drying out, the harsh terrestrial environment and predators. The allantois helps to exchange and store waste products produced by the embryo, a structure not needed when eggs are immersed in water and waste products can diffuse freely through a weak and porous shell.

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