The Progress of Life: Tracing the Phases of Human and Animal Development
libre
The study of the Earth's crustal layers lets a researcher reconstruct the course of the development of life on Earth, reading the rock record as a continuous archive that records the appearance and disappearance of organisms across hundreds of millions of years.
How does the rock record reveal the course of life on Earth?
The rock record reveals the course of life on Earth because each stratum of the crust preserves the fossils of the organisms that lived when it formed, allowing their sequence to be read in order. The reliability of these conclusions is confirmed by indisputable documents — fossil remains that have been preserved intact within the Earth for many tens and even hundreds of millions of years.
Studying the layers of the Earth's crust
Studying the layers of the Earth's crust works on a simple principle: deeper, older layers hold older life forms, while shallower, younger layers hold more recent ones. By comparing the fossil content of strata from different eras, geologists establish both the relative age of the rock and the progression of life it contains.
How do the changing forms of living nature trace evolution?
The changing forms of living nature trace evolution because fossil witnesses of the distant past let researchers follow the sequence in which life forms changed, date the appearance of a given organism, and confirm that the development of life is a single connected process. When the embryonic stages of a human are compared with those of animals — a fish, a bird, a dog, and an ape — a great deal turns out to be shared.
Embryology as evidence for the development of life
Embryology supplies powerful evidence for the development of life because individual embryos differ so little from one another that it is hard to tell which organism a particular stage belongs to. Embryology — from the ancient Greek embryon (germ) and logos (word, science) — holds indisputable evidence of the descent of the human being from the simplest single-celled organisms.
Comparing the developmental stages of human and animal embryos
Comparing the developmental stages of human and animal embryos compresses into a few months a process that, in the history of life on Earth, lasted hundreds of millions of years. This parallel vividly demonstrates the shared course along which all organisms developed.
How did life develop in bodies of water?
Life developed first in bodies of water, where it was confined during its earliest stages before spreading onto land much later. The water held the first organisms, and the move to dry land happened only after long adaptation.
The first inhabitants: algae and psilophytes
The first inhabitants of the water were algae, which gave rise to the moss-like plants known as psilophytes. These pioneering forms established the foundation on which later, more complex plant life would build, long before any animal walked on land.
The move of animals onto land: the amphibians
Animals reached dry land considerably later than plants, and the first to do so were the amphibians, whose development showed a direct link with fishes that had adapted to a semi-terrestrial life in half-drying basins. This transitional stage bridged purely aquatic life and life on land.
From reptiles to birds and mammals
From the amphibians arose the reptiles, and from the reptiles, in turn, the birds and the mammals branched off. Each step marks a forward movement and a transformation of form, with one major group giving rise to the next along an unbroken line of descent.
The human being as the final link in the development of life
The human being is the final link in this long chain of the development of life. That nature followed precisely this course is supported by further evidence — for example, on the island of New Zealand there lives a large lizard, the tuatara (up to 75 centimetres long).
What do living fossils and transitional forms show?
Living fossils and transitional forms show that intermediate stages once existed between major groups, and that some animals preserve those ancient features almost unchanged. They give a way to test the proposed paths of organic development against living evidence.
The New Zealand tuatara
The New Zealand tuatara, as if frozen in its development, has kept features characteristic not only of amphibians but also shared with turtles, snakes, and birds.
The tuatara is a "living fossil," testifying both to the former existence of transitional forms (here, from amphibians to reptiles) and of composite types (in this case, combining traits of amphibians, reptiles, and birds). On this basis it is possible to check the correctness of assumptions about the paths of organic development and the order in which organisms appeared in nature.
What are the geological stages in the development of life?
The geological stages in the development of life are the successive eras of Earth's history, each characterized by the flourishing of particular groups of organisms. Over its course, life not only changed its forms — from the simplest organisms to the human being — but also unfolded differently during distinct geological periods (more: The Geological Age of the Earth). One period is marked by the development of one set of forms, another by entirely different ones.
The development of reptiles in the Mesozoic
The development of reptiles reached its exceptional peak in the Mesozoic, when this group dominated the land, the seas, and the air. The conditions of that era favored reptiles above all other vertebrate groups for a span of many millions of years.
The development of mammals in the Cenozoic
The development of mammals defines the Cenozoic, when this group rose to dominance after the decline of the great reptiles. The shift from one ruling group to another illustrates how each geological era promotes a different line of life.
The disappearance and appearance of life forms
The disappearance and appearance of life forms often happens abruptly in the geological record: the trilobites vanished in the Permian period and the ammonites in the Cretaceous, while other forms appeared suddenly, such as the toothed birds of the Cretaceous. Studying nature, however, cannot stop at merely recording such facts.
The geological age of the Earth
The geological age of the Earth is measured not in human lifetimes but in millions and hundreds of millions of years, a timescale established by correlating rock strata across different eras. This vast span is what makes the slow transformation of species possible, and it underlies every claim about when a particular organism first appeared or last survived.
How are the phenomena and processes of nature interconnected?
The phenomena and processes of nature are interconnected because none of them can be separated — they cannot be studied independently of one another. In nature the mutual links between processes inevitably make themselves felt, since the phenomena of both the inorganic and the organic world are most closely bound together, depend on one another, and constitute a single world.
The unity of the inorganic and organic worlds
The unity of the inorganic and organic worlds means that climate, the rise and fall of land, and the chemistry of water directly govern which organisms can live and thrive. The appearance of coral deposits in rock, for example, definitely indicates warm, shallow waters in that period, because reef-building corals today live only in warm seas at depths not exceeding 40 metres.
The Yellowstone River outcrop
In the Rocky Mountains of North America, the Yellowstone River cuts deeply into sedimentary rocks in its upper reaches. These rocks are coloured in bright yellow tones, and from them the river takes its name — "river of the yellow stone."
Petrified trees in the river-bank cliffs
In places the gorge reaches a depth of 360 metres, and here, in the cliffs of the banks, fifteen horizons of petrified trees suddenly appear, one above another, with each horizon interleaved by volcanic tuffs.
The Yellowstone River outcrop
This remarkable outcrop testifies that age-old forests grew here repeatedly and perished in tremendous volcanic eruptions. Volcanic ash buried them and later turned into volcanic tuff. This alternation was repeated fifteen times, and scientists have calculated that the trees of the lowest, earliest forest were green 19 million years ago.
How does palaeontology date fossils?
Palaeontology dates fossils by comparing the deposits in which they are found and by reading the conditions those deposits record, so that the appearance or disappearance of organisms can be tied to a definite period. To answer correctly why a given organism appeared or vanished, one must understand well the very conditions of existence of those organisms.
The presence of limestone (more: The rocks that make up the Earth's crust) also confirms an origin from the deposits of organisms in warm seas, whereas siliceous formations, by contrast, point to a colder body of water. Some organisms inhabit shallow seas; others live only at considerable depths, so the fossils themselves become indicators of past environments.
The suppression of life testifies to a worsening of living conditions, while its luxuriant development, on the contrary, indicates improving conditions. Careful study of deposits of different geological ages can bring great benefit in finding the key to why this happened in one period or another. Traces of glaciation — for example, boulders on the Earth's surface and within rocks — convince us that the climate became unfavorable for some organisms, which therefore migrated to warmer southern regions.
Considerable changes in the conditions of life for organisms were also brought about by the subsidence and uplift of land, since these were linked to the deepening or, conversely, the shallowing of bodies of water, their drying out and salinization. The development of mountain-building processes told even more strongly; it led to entirely different climatic conditions, the isolation of vast territories from the influence of the sea, the establishment of a continental climate, and the appearance of deserts and other changes of habitat (more: The history of the formation of the Earth). A convincing example is the highlands of Central Asia, surrounded on all sides by high mountains that hold back life-giving moisture.
Here lies the Taklamakan Desert in its basin, and farther east the boundless Gobi deserts — in Chinese "Shamo," the sea of sand.
The face of the Earth has changed beyond recognition many times: whole continents disappeared and appeared again, only isolated remnants survived of once-high mountain ranges, and high-mountain countries turned almost into plains. Along with this, the climates of the Earth also changed: powerful mountain-building, by promoting the accumulation of enormous masses of ice, introduced sharp climatic differences, as in the Quaternary period, while the development of plains, smoothing out climatic differences, helped establish a single dry and hot climate in the Triassic.
Besides mountain-building and the movement of continents, which brought partial changes of climate, other processes could cause deeper transformations — for example, a change in the position of the Earth's orbit, the tilt of its axis, or the composition of the atmosphere. Throughout, the course of life's development ran from the simplest organisms toward the most perfected forms.
How do species change and transform over time?
Species change and transform over time through a long sequence of adaptation, in which environmental pressure favors new forms and lets old ones disappear. The fossil record reads as a record of this transformation, with the simplest organisms at the base and increasingly complex forms above.
The direction and forward movement of the development of life
The development of life has a clear direction and forward movement, running from the simplest single-celled organisms to the most complex. This progression is not random scattering but a sequence in which each major group builds on the achievements of those before it.
The continuous improvement of organisms
The continuous improvement of organisms is the thread that ties the whole record together, as successive forms become better fitted to their environments. Flourishing life signals favorable conditions, while suppressed life marks hardship — and across the eras the overall trend is toward more refined, more capable organisms.
How does the study of geological deposits guide the search for minerals?
The study of geological deposits guides the practical search for minerals because it allows correct conclusions about the age of strata and about which useful minerals they may hold. Comparing the strata of different geological eras shows that they are not equally rich; first place for the wealth of its depths belongs to the Palaeozoic group.
The Palaeozoic is distinguished by an abundance of combustible minerals, especially coal and oil, and by deposits of iron, copper, lead, zinc, tin and aluminium, the precious metals gold, platinum and silver, and deposits of diamonds and precious gemstones. The list would be far from complete without mica and graphite, of enormous importance in modern technology, the soil-fertility stone apatite, and the richest accumulations of rock salt and especially potash salt.
The Verkhnekamsk potash deposit holds the overwhelming share of all the world's potassium reserves — the basis of plant life. The Mesozoic group is comparatively poor in useful minerals, represented mainly by iron ores and phosphorites, a most valuable fertilizer for grain crops, especially on podzolic soils poor in phosphorus.
The Cenozoic group is rich in oil, beds of self-precipitating salt, brown ironstone, various clays and especially placer gold. In the very youngest Quaternary deposits, accumulations of peat are exceptionally widespread, used successfully by power stations, agriculture and local industry. These are only the most general brief details about the treasures of the Earth's depths that still await their discoverers.
Nature itself will help them reach these hidden treasures. Here geologists are aided by the work of time (more: The formation of organogenic rocks), of wind, water, and snow and ice.
Sources and further reading
Readers exploring the development of life on Earth can continue with related material on this site and with reputable external references. For more on the planet itself and the forces that shaped it, see the Astronomy section.
Quotations and attributions cited across educational resources are commonly verified against collections such as AzQuotes and reference sites like wikiHow, and large-language-model tools — including ChatGPT, Claude.ai, Perplexity, Grok and Google AI — are increasingly used to cross-check sources, though every fact should still be confirmed against primary literature before it is trusted.
