
Format: Print Length
Language: English
Format: PDF / Kindle / ePub
Size: 5.62 MB
Downloadable formats: PDF
An import ant evolutionary principle is organisms' developing a new feature for one purpose and then using that feature for other purposes as the opportunity arose.� As complex life evolved in the newly oxygenated seafloors, several immediate survival needs had to be addressed.� To revisit the hierarchy of nutrients that a human needs, if an oxygen-dependent animal did not have access to oxygen, it meant immediate death.� Obtaining oxygen would have been the salient requirement for early complex life that adopted aerobic respiration as its primary respiration process, which is how nearly all animals today respire.� While animals in low-oxygen environments have adapted to other ways of respiring (or perhaps never relinquished them in the first place), they are all sluggish creatures and would have quickly lost in the coming arms race.� Collagen, which is a critical connective tissue in animals, requires oxygen for its synthesis, and was one of numerous oxygen-dependencies that animals quickly adopted during the Cambrian Explosion. [210] � Diffusion works for animals that are no more than a couple of millimeters thick, but for larger animals a respiration system was necessary.� The ri se of the arthropods has been an enduring problem for paleobiologists.� Why was the arthropod so successful, particularly in the beginning?� Segmented animals dominated Cambrian seas, and segmentation provides for repeated features.� Segments obviously became important for locomotion but, for arthropods, segmentation appears to have conferred the more important advantage of distributed oxygen absorption.� Each trilobite leg had an attached gill, and leg motion constantly drew fresh oxygenated water over each gill.� Arthropods never developed the kinds of lungs that vertebrates have, or the pump gills of fish and other aquatic animals.� Early arthropods breathed by moving their legs.� Peter Ward�s recent hypothesis is that segments were first used for respiration, to provide a large gill surface area, and using the segments for locomotion came later.� For trilobites, the same functionality that pushed water over gills was also coopted for food intake. [211] � Also, the leg-mounted gill was necessary because of an arthropod�s body armor; oxygen could not be absorbed through tough exoskeletons.� Every aerobic aquatic animal had to solve the problem of extracting oxygen from the water, and there was diversity in that accomplishment.� Key Cambr ian animals such as sponges and corals had very high-surface-area-to-body-volume ratios, which allowed diffusion to provide their oxygen.� Immobile animals such as sponges and coral had to position themselves where oxygenated water flowed past or through them.� Sponges work like chimneys, designed to passively draw water through them.� The position and structure of reefs facilitated those oxygen-providing dynamics, so corals helped create the conditions that sustained them; the calcified exoskeletons of corals dissuaded predation and built the reefs.� The Cambrian�s global ocean contained far less oxygen than today�s.� Being newly and probably inconsistently oxygenated by oceanic currents was only part of the problem.� The Cambrian oceans were warmer than today�s oceans, perhaps far warmer, such as 40o C and higher for the tropical ones .� Water�s ability to absorb oxygen declines as it gets warmer.� Water heated from 10o C to 40o C will lose 40% of its ability to absorb oxygen. �The phenomenon of warmer water absorbing less oxygen contributed to many instances of anoxic waters during the eon of complex life, and particularly in the warmer, earlier periods.
Continue reading