When Did Life on Earth Begin? Exploring the Origin of the First Life Forms
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Science still cannot say, even approximately and even with an error of millions of years, exactly when life first appeared on Earth. What is beyond dispute is that living matter changed over the hundreds of millions of years of Earth's existence, shifting in response to environmental conditions and the conditions under which organisms lived.
How did plant and animal organisms develop?
Comparing a plant organism with an animal organism reveals profound differences between them. Yet as you move from higher forms to lower ones, from the more highly organized to the less organized, those differences gradually fade. The simplest representatives of animals and plants come so close to one another that dividing them is arbitrary, and drawing a sharp boundary between them is not possible.
This convincingly points to the unity of life. Life developed and perfected itself gradually. Through continuous change, new plant and animal organisms appeared, better adapted to new habitats. The plant and animal world familiar to us today is only one stage in that vast, time-spanning process of the development of life, a process that began very long ago.
How do the layers of Earth's crust tell the story of life's origins?
Earth's past is told eloquently by the layers of the crust, with the remains of various organisms preserved within them, by the rocks that make up those layers, by their arrangement and other features (for more detail see How life arose in the ancient eras of the Earth). These layers are like the pages of a special, captivating book about the life of the Earth.
You only need to know how to read its worn, sometimes badly scattered pages.
In a deep ravine or on a riverbank you can find shells unusual in appearance and shape, imprints of plants and animals on stone, stones resembling honeycombs or little ram's horns, as well as stone tubes pointed at one end and varying in size and thickness.
These tubes somewhat resemble fragments of stone fingers. Because of that resemblance they are commonly nicknamed "devil's fingers."
You may also be lucky enough to find oddly shaped teeth, bones, and even whole skeletons, as well as imprints — sometimes of enormous size — of animals no one has ever seen.
The rocks making up the masses of the crust can be no less remarkable than the fossil remains of organisms found within them. In some places our attention is caught by blue, red, and black clays; in others by black, red, and green sandstones, white and green sands, and limestones at times packed full of the remains of various organisms.
Why do different layers contain different fossils?
Investigators of nature noted long ago that different layers contain the remains of different organisms. In some layers — for example near Saint Petersburg — one is struck by the abundance of small flat shells called "obolus," roughly the size of a two-kopeck coin (in Greek "obolos" was a small coin, an obol); in other layers, such as those near Moscow, "devil's fingers" abound.
An abundance of "devil's fingers" in the layers
From this came the conclusion that these layers formed at different geological times, when precisely these organisms were widespread in marine basins. The obolus inhabited the ancient Silurian Sea, which, according to geologists, arose roughly 360 million years ago and existed for some 40 million years.
That sea covered an enormous area, stretching from the eastern borders of Western Europe to the Aral Sea in the east, and roughly from the latitude of Tula in the north to the Caucasus Mountains in the south. Present-day seas, such as the Black Sea, likewise cast up vast masses of all kinds of shells. On the "golden" beach at Yevpatoria you would be struck by the abundance of shells.
Local craftsmen skillfully decorate their simple souvenirs with these shells — little boxes, photo frames, and various trinkets. Beyond its decorative use, shell material serves well in place of ballast sand for railway beds. Thick masses of Black Sea shells became the source material for forming layers of shell rock — an excellent building material that is easy to work.
What is a "devil's finger" really?
The "devil's finger" has an equally interesting history. The devil is invoked here only out of ignorance: the object is nothing more than fragments of the internal shell of an ancient cephalopod mollusk, the belemnite, which lived in the distant Mesozoic era roughly 185 million years ago.
The animal's name comes from the ancient Greek word "belemnon" — an arrow, whose pointed head, on the whole, resembled a "devil's finger."
What animals descend from the belemnites?
The few descendants of the belemnites — cuttlefish and the giant monsters known as octopuses, or "sprouts" — live in present-day seas, both cold and warm, both near the shore and at great depths (down to 3,500 meters). Most cephalopods are predators; sometimes they reach 17 meters in length, of which 6 meters belong to the animal's body and the rest to the tentacles — "legs" numbering up to ten.
Cephalopods swim by a special method: with a powerful contraction of the body's muscles they expel a jet of water from the mouth opening. The recoil sends the animal shooting forward like a torpedo, so that it seems to swim backward. When in danger, some cephalopods release the contents of a special ink sac and, behind that murky screen, become invisible to their enemies.
The contents of the ink sac are used to make the famous China ink and the brown pigment sepia. Many cephalopods, especially cuttlefish, are eaten (in China) both fresh and dried. The "devil's finger" itself lay in the tail part of the animal and gave the predator its speed of movement.
What were the ancient seas like?
Ancient cephalopods lived in abundance in the Cretaceous Sea, which during the first half of the Cretaceous period flooded a broad belt along the Ural ridge, reaching westward in a deep gulf to the meridian of Tver–Kaluga, while in the second half it occupied almost the entire southern half of the European part of Russia, down to the southern borders with Turkey and Iran.
In this southern region of the Cretaceous Sea, the Main Caucasus ridge was already emerging as a rocky island.
How do matching fossils help date the layers?
When the layers of regions far apart from one another — for example near Moscow and near Ulyanovsk — are found to hold an abundance of "devil's fingers" or other identical organic remains, this convincingly shows that these layers formed at one and the same geological time — in other words, in the same geological period, epoch, age, and so on.
What do the layers of the Quaternary period reveal?
The study of the layers of the crust formed over the last million years closest to us can give us interesting material. This geological period, which continues to the present day, is called the Quaternary period.
In the very topmost layers of the Lower and Middle Volga region — for example in the Astrakhan, Volgograd, Saratov, and Kuibyshev provinces, and especially in the Trans-Volga area — there are shells resembling those that live in the Caspian Sea today.
From the finds of these shells it was possible to establish the boundaries of the vast Aral–Caspian Sea that once existed. Volgograd and Saratov now stand on what was its bedrock shore. Researchers can even establish precisely that the sea's narrow northern gulf ran along the high right bank of the Kama far to the northeast.
The sea was still like this about 100,000 years ago, when most of the European territory of Russia lay beneath the cover of the great glaciation, and the thickness of the ice reached, geologists believe, up to two kilometers.
In deeper layers of the Volga region one finds the bones of bison-oxen, wild horses, enormous camels, the mammoth, the giant deer, the woolly rhinoceros, the cave lion, and other now-vanished animals. The deeper we penetrate into the layers, the more often we encounter bones of animals that differ more and more from present-day representatives of the animal world.
Studying the petrified remains of life from bygone epochs, geologists turn over, as it were, the stone pages of nature's great book. Yet that book often gives no exhaustive answer: many pages are missing, since far from all the organisms that existed in past epochs of our planet's life left their trace in stone.
Of the long chain of life — from the emergence of living matter to its most perfect form, the human being — only separate fragments survive, and many links of that chain are missing. The most ancient layers of the crust, greatly altered during its formation, contain almost no signs of organic life.
How are fossil organisms formed?
More distinct traces of organisms begin to appear in rocks that formed from the sediments of ancient water bodies. Organisms and their skeletons buried in these sediments gradually turned to stone under favorable conditions — in other words, they became mineralized.
Their organic matter was replaced from solution by mineral matter — for example calcium carbonate, silica, and other substances. In this way various petrified shells, bones, pieces of wood, and even whole tree trunks were formed.
If you grind a thin, transparent plate (thinner than a sheet of paper) from a piece of petrified wood — a so-called thin section — then under the microscope we can clearly see the internal structure of the ancient wood. Sometimes it is not the shells or parts of the plant themselves that survive, but only their imprints — for example the imprints of plant leaves.
One also finds casts formed from material that filled a shell and later hardened. These produce what geologists call "internal molds." They resemble metal castings made in a particular mold. When the shell itself dissolves, a cast of its outer form, or "external mold," results.
The environment in which the remains of animals were preserved determined how well they survived: in coarse-grained sands the remains of animals dissolved away in circulating waters, in clays they were crushed, and in metamorphic rocks they disappeared completely. Only fine-grained silty sediments, peat, natural asphalt, and especially the resin of conifers ensured exceptional preservation of organic remains.
Insects, for example, and flowers that fell into liquid tree resin millions of years ago survived whole, without the slightest change, as if alive. How can this be explained?
The point is that the resin gradually hardened and turned to stone, becoming amber — a semiprecious golden stone, often completely transparent. From amber people make beads, cigarette holders, brooches, and the like. Various insects, especially ants, are often found in amber.
Here is what Lomonosov wrote about these curiosities about 260 years ago:
Strolling in the shade of a poplar, an ant got its leg stuck fast in clinging resin. Though in life it was despised by people, after death, in amber, it became precious to them.
Geological finds, especially in the old days, were by no means always correctly identified and described. There were also unforgettable absurdities. In one Spanish cathedral in the 17th century, for instance, a mammoth's molar was revered as the undoubted tooth of a saint. Those suffering from toothache would press themselves to the mammoth tooth and, on the whole, brought a tidy income to the "holy fathers."
Let us note the approximate dimensions of a mammoth tooth: the length of the root is 12 centimeters, the length of the chewing surface is 14 centimeters, and its width is 7 centimeters. A person is supposed to have thirty-two teeth (when the set is complete). Just how large, then, was the saint's mouth, judging by the incontrovertible evidence of the relic itself?
It should be noted that legends of giants twenty times taller than a human being appeared even in old "scientific" treatises of that time. Still graver cases occurred with geological finds. The skeletal imprint of an ancient lizard was, for example, declared — with the blessing of the "learned men" of the first quarter of the 18th century — to be the skeleton of a human being who had drowned during the "great flood."
Gradually the fog of mystery lifted from various wondrous finds, and by the end of that same 18th century scholars who carefully studied the structure of the animal organism debunked the "witness of the great flood," discovering in it an unmistakable resemblance to a lizard. Besides fossils and imprints, direct traces of the most ancient life or of natural phenomena are also often found on stone.
Such are, for example, the tracks of the limbs of the most ancient animals, the trails left by crawling worms, the imprints of raindrops, ripple marks, and so on.
Initially these were imprinted on soft ground, then gradually hardened and turned to stone.
