A New History of Life: The Radical New Discoveries About the Origins and Evolution of Life on Earth by Peter Ward & Joe Kirschvink

Author:Peter Ward & Joe Kirschvink [Ward, Peter & Kirschvink, Joe]
Language: eng
Format: epub
Tags: Science, Life Sciences, Biology, Evolution, Earth Sciences, General
ISBN: 9781608199082
Google: DA8bBQAAQBAJ
Publisher: Bloomsbury Publishing
Published: 2015-04-06T12:00:00+00:00

OXYGEN AND TEMPERATURE, REPRODUCTION AND THERMOREGULATION

At this point we can summarize and discuss the variables in land animal reproduction and try to relate these to generalizations about both oxygen levels and temperature. There are two possible strategies, as we have seen: egg laying or live birth. In the egg case, the eggs are either covered with a calcareous shell cover or a softer, more leathery shell cover. Today, all birds utilize calcareous eggs, while all living reptiles that lay eggs use the leathery covering. Unfortunately, there is little information about the relative oxygen diffusion rates for leathery—or parchment—eggs compared to calcareous eggs.

The utilization of egg laying or live birth has important consequences for land animals. The embryos developed by the live-birth method are not endangered by temperature change, desiccation, or oxygen deprivation. But the cost is the added volume of the parent, which must invariably make her more vulnerable to predation in addition to needing more food than would be necessary for the adult alone. Egg layers are not burdened with this problem, but have the trade-off of a less safe environment—the interior of an egg outside of the body—that leads to enhanced embryonic death rate through predation or lethal conditions of the external environment.

Before the end of the Mississippian period three great stocks of reptiles had diverged from one another to become independent groups: one that gave rise to mammals, a second to turtles, and a third to the other reptilian groups—and to the birds. The fossil record shows that there are many individual species making up these three. A relatively rich fossil record has delineated the evolutionary pathway of these groups. It has also required a reevaluation of just what a “reptile” is. As customarily defined, the class Reptilia includes the living turtles, lizards, and crocodiles. Technically, reptiles can now be defined by what they are not: they are amniotes that lack the specialized characters of birds and mammals. Less appreciated is that all three of these lineages originated in a world with extensive glaciation and very high oxygen. It is the assumption here that coming from a cold but high-oxygen world would have affected many aspects of the biology of these animals. Let us look at some of these characteristics.

One of the enduring questions about the history of life concerns the history of thermoregulation in animals. There are three distinct kinds: endothermy (warm-blooded), ectothermy (cold-blooded), and a third category (homeothermic) that is essentially neither of the others, and is associated with very large size. The evolution of each of these has long attracted scientific scrutiny, with thermoregulation pathways—most important, the question of whether or not dinosaurs were warm-blooded—being the most discussed and controversial of all. The fact that each of these characteristics is primarily either physiological or involved body parts that only rarely leave any fossil record (such as fur) is in large part responsible for the controversies.

We know that all living mammals and birds are warm-blooded, with the former having hair and the latter feathers, just as we know that all living reptiles are cold-blooded, with neither hair nor feathers.

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