tightly interconnected by a variety of elaborate links. Their mutual coordination and control was greatly increased by the very early creation of nervous systems, and by 620 million years ago tiny animal brains had evolved.
The ancestors of plants were thready masses of algae that dwelled in sunlit shallow waters. Occasionally their habitats would dry up, and eventually some algae managed to survive, repro-
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duced, and turned into plants. Those early plants, rather like today’s mosses, had neither stems nor leaves. To survive on land it was crucial for them to develop sturdy structures so that they would not collapse and dry out. They did so by creating lignin, a material for cell walls that enabled plants to grow sturdy stems and branches, as well as vascular systems to draw water up from the roots.
The major challenge of the new environment on land was the shortage of water. The creative answer of plants was to enclose their embryos in protective, drought-resistant seeds, so that they could wait with their development until they found themselves in an appropriately moist environment. For over 100 million years, while the first land animals, the amphibians, evolved into reptiles and dinosaurs, lush tropical forests of “seed ferns”—seed-bearing trees that resembled giant ferns—covered large portions of the Earth.
About 200 million years ago glaciers appeared on several continents, and the seed ferns could not survive the long, cold winters. They were replaced by evergreen conifers, similar to our present- day fir and spruce, whose greater resistance to cold allowed them to survive the winters and even to expand into higher alpine regions. One hundred million years later flowering plants whose seeds were enclosed in fruits began to appear.
From the beginning these new flowering plants coevolved with animals, who enjoyed eating their nutritious fruits and in exchange disseminated the undigested plant seeds. These cooperative arrangements have continued to develop and now also include human growers who not only distribute plant seeds, but also clone seedless plants for their fruits. As Marguhs and Sagan observe, “Plants indeed seem very adept at seducing us animals, having tricked us into doing for them one of the few things we can do that they cannot: move.” 44
Conquering the Land
The first animals evolved in water from globular and wormlike masses of cells. They were still very small, but some of them
formed communities that collectively built huge coral reefs with their calcium deposits. Lacking any hard parts or internal skeletons, the early animals completely disintegrated at death, but a hundred million years later their descendants produced a wealth of exquisite shells and skeletons that left clear imprints in well- preserved fossils.
For animals, the adaptation to life on land was an evolutionary feat of staggering proportions, requiring drastic changes in all organ systems. The greatest problem in the absence of water, of course, was desiccation; but there were a host of other problems as well. There was enormously more oxygen in the atmosphere than in the oceans, which required different organs for breathing; different types of skin were necessary for protection against unfiltered sunlight; and stronger muscles and bones were needed to deal with gravity in the absence of buoyancy.
To ease the transition to these totally different surroundings, animals invented a most ingenious trick. They took their former environment with them for their young. To this day the animal womb simulates the wetness, buoyancy, and salinity of the ancient marine environment. Moreover, the salt concentrations in the mammal blood and other bodily fluids are remarkably similar to those in the oceans. We came out of the ocean more than 400 million years ago, but we never completely left the seawater behind. We still find it in our blood, sweat, and tears.
Another major innovation that became vital for living on land had to do with the regulation of calcium. Calcium plays a central role in the metabolism of all nucleated cells. In particular it is crucial to the operation of muscles. For these metabolic processes to work, the amount of calcium must be kept at precise levels, which are much lower than the calcium levels in seawater. Therefore marine animals from the very beginning had to continually remove all excess calcium. The early smaller animals simply excreted their calcium waste, sometimes piling it up in enormous coral reefs. As larger animals evolved, they began to stockpile the excess calcium around and inside themselves, and these deposits eventually turned into shells and skeletons.
As the blue-green bacteria had transformed a toxic pollutant,
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oxygen, into a vital ingredient for their further evolution, so the early animals transformed another major pollutant, calcium, into building materials for new structures that gave them tremendous selective advantages. Shells and other hard parts were used to fend off predators, while skeletons emerged first in fish and subsequently evolved into the essential support structures of all large animals.
Around 580 million years ago, at the beginning of the so-called Cambrian period, there was such a profusion of fossils with beautiful clear imprints of shells, rigid coats, and skeletons that paleontologists believed for a long time that these Cambrian fossils marked the beginning of life. Sometimes they were even viewed as records of God’s first acts of creation. It is only within the last three decades that the traces of the microcosm have been revealed in so-called chemical fossils. 45 These show conclusively that the origins of life predate the Cambrian period by almost three billion years.
Evolutionary experiments with calcium deposits led to a great diversity of forms—tubular “sea squirts” with spinal columns but no bones, fishlike creatures with external armors but without jaws, lungfish that breathed both water and air, and many more. The first vertebrate creatures with backbones and a braincase shielding the nervous system probably evolved around 500 million years ago. Among them was a lineage of fish with lungs, stubby fins, jaws, and a frog-like head, which crawled along the shores and eventually evolved into the first amphibians. The amphibians— frogs, toads, salamanders, and newts—are the evolutionary link between water and land animals. They are the first terrestrial vertebrates, but even today they begin their life cycle as waterbreathing tadpoles.
The first insects came ashore around the same time as the amphibians and may even have encouraged some fish to feed on them and follow them out of the water. On land the insects exploded into an enormous variety of species. Their small size and high reproductive rates allowed them to adapt to almost any environment by developing a fabulous diversity of body structures and