composed of chemically bound oxygen and hydrogen, bombarded the Earth, releasing these chemicals as water vapor and increasing the water resources of our planet. High up in the
TIME LINE:
| 4.6 bya | Explosion and interstellar cloud of gases |
| 4.5 bya | Earth forming |
| 3.8 bya | First water |
| 3.4 bya | First life, photosynthetic bacteria (single cell) |
| 1 bya | First sexual reproduction |
| 750 mya | Multicelled organisms (cell colony) |
| 500 mya | Hydra and starfish (radial symmetry) |
| 400 mya | Bony fish, first land plants and animals (bilateral symmetry) |
| 350 mya | Amphibians and insects radiate; coniferous trees |
| 280 mya | Reptiles radiate |
| 250–200 mya | Pangaea and Panthalassa |
| 225–65 mya | Dinosaurs |
| 180 mya | First small mammals and birds appear (reptiles rule) |
| 135 mya | Angiosperms (flowering plants) widespread |
| 65 mya | Continents in current positions |
| 50 mya | Early primates radiate (hunkering) |
| 22 mya | Early apes (brachiation) |
| 5 mya | Hominoid/hominid (ape/human) split |
| 3.7 | Homo erectus (bipedal alignment). |
| 2 mya– | |
| 10,000 ya | Glaciers (widespread 65,000 ya) |
| 1.8 mya | First stone tools |
| 130,000– | |
| 75,000 ya | Homo sapiens |
Dates indicate longer, overlapping periods of time; they also change with new findings. bya, billion years ago; mya, million years ago; ya, years ago
The first time I went snorkeling forever changed my view of life on land. There, below the undulating surface film of the Caribbean, was an underwater world as complex and diverse as any I had known. Trigger fish with their tiny mouths, colorful parrot and butterfly fish, angelfish with vertical stripes, serious-looking grouper and schools of ephemeral bonefish moved through the waters. Amid the living corals, sea plants, and algae, undulating sting rays, giant sea turtles, creeping starfish, and sand dollars inhabited this unique terrain. Later I learned that familiar mountains and valleys are also present in the sea floor, a vast expanse covering two-thirds of the globe. In fact, John McPhee reminds us, in Annals of a Former World, that what is topsoil becomes ocean floor, and what is ocean floor becomes topsoil in a continuous rhythm of change.
Marine protozoa, radiolaria.
Earth’s evolving atmosphere, it was cool enough for water vapor to condense into liquid water; it is possible that rain occurred over millions of years, creating the first oceans. Sedimentary rocks, marking the earliest presence of water on the earth, have been dated at 3.8 billion years.
Molecules of many chemicals were washed out of Earth’s surface rocks, and shallow pools eventually filled with diverse groupings. One theory suggests that, when conditions such as temperature were right, a unique combination occurred that included molecules called amino acids; another theory postulates that amino acids came to Earth as a component of meteorites. Whatever their origins, amino acids became part of complex molecules that could make copies of themselves, and these molecules were incorporated into cells that could then replicate.
The elements that compose amino acids and cell structure are actually part of larger cycles. Carbon, nitrogen, and phosphorus are crucial to the existence of all living things. Carbon is stored primarily in deep oceanic sediments (and is essential for building physical structure and storing energy); nitrogen is present in gaseous forms in the atmosphere (and is a large component of many enzymes that break down carbon compounds and release energy for the body’s use); and phosphorus is bound in the continental crust, where it weathers and becomes available for uptake (for the building blocks of DNA and cell membranes). To exist, all organisms depend on consistent ratios of carbon, nitrogen, and phosphorus cycling through the land, water and atmosphere.
Cells are the structural building blocks of all living beings. As the first living organisms, they exhibited the basic characteristics of life: the ability to reproduce, metabolize, and respond to changes in the environment. Earliest life forms were single-celled prokaryotes, similar to modern-day bacteria. The geological record shows that, about 3.4 billion years ago, self-feeding (autotrophic) prokaryotes, like blue-green algae, evolved; they could make their own food through photosynthesis, a metabolic process using carbon dioxide, water, and sunlight to form simple sugars for energy storage. From these unique, single-celled, photosynthetic bacteria, more complicated forms evolved, such as plants that could also make their own food, converting sunlight to organic matter by photosynthesis and releasing oxygen. This process once again changed the chemical balance of the atmosphere.
According to the fossil record, bacteria were the only organisms for the first two-thirds of Earth’s history, dating back over 3.4 billion years ago. They reproduced exact copies of themselves by cell division. Sexual reproduction, combining genetic material from two parents, began around 1 billion years ago. Multicellular plants and animals first appeared 750 million years ago. The first land plants and insects evolved around 400 million years ago. The first birds and mammals developed over 180 million years ago. It has been 65 million years since the dinosaurs disappeared. Glaciologists tell us that over the past 2 million years and as recently as 10,000 years ago, large glaciers covered the North American and Eurasian continents and smaller glaciers occupied alpine valleys. Evolutionists remind us that hominids have walked the Earth for a mere 5 million years, with anatomically modern humans, Homo sapiens, appearing in Africa around 130,000 to 75,000 years ago. Reflecting on underlying patterns of temporal evolution offers a perspective of other than human scale to our present experience of place.
The theory of plate tectonics, or continental drift, has been generally accepted since the 1960s, suggesting that Earth’s crust is divided into about twelve continental plates that float on the partially molten magma of the mantle and move with convection currents. It is proposed that 400–500 million years ago there were continental blocks (paleocontinents) whose movements and collisions are recorded in mountain belts. Although the specific locations of these early blocks are conjecture, fossil records and rock compositions help geologists configure their possible locations. Geologists propose that some 250 million years ago tectonic plates collided to form a single massive continent, called Pangaea, that stretched from pole to pole, surrounded by a universal ocean, Panthalassa. Around 200 million years ago the supercontinent separated into land masses that would eventually become the northern continents (Laurasia) and a southern supercontinent (Gondwana).
The northern Atlantic ocean was formed around 180 million years ago, when the northern continents formed a crack, followed by sea floor spreading, which separated Eurasia and the Americas. The southern Atlantic ocean was created around 130 million years ago when South America broke away from Africa. By