Basics of Evolution - O.P. Korzh - 2006

Part III. PATHS OF LIFE'S DEVELOPMENT

Chapter 17. Geochronology

17.3. Geochronology of Life

Scientists suggest that at the time of Earth's formation (about 4,600 million years ago), its surface Temperature was quite high, reaching 4,000–8,000 °C. Gradually, the planet cooled, refractory metals and carbon condensed, and the Earth's crust was formed—barren and rugged at the time due to volcanic activity and constant tectonic shifts. Today, most researchers believe that the Earth's atmosphere-ocean system had a different composition back then: light gases (hydrogen, oxygen, helium, nitrogen, and argon) escaped the atmosphere because the gravitational field was not strong enough to retain them; the atmosphere contained ammonia, carbon dioxide, and

methane. Until the crust's temperature dropped below 100 °C, Water existed as vapor. Afterwards, Condensation of the primordial oceans began near the poles, the planet's surface became exposed to solar radiation, and the first sedimentation processes took place.

Class="center">Table 17.1. Geochronological Periodization (according to various authors)

The cooling of the Earth and prebiological evolution lasted for over a billion years. The entire METABOLISM/13.html">History of the Earth's development is divided into large time intervals—eons, eras, periods, epochs, and ages—the delineation of which is associated with geological and climatic events on our planet (Table 17.1). Changes in the biotic environment (formation of continents and the World Ocean, mountain-building processes, climate change, etc.) also influenced the Evolution of the organic world.

The course of life's evolution on the planet is divided into two major eons: the Cryptozoic and the Phanerozoic. Although the duration of the Cryptozoic exceeds that of the Phanerozoic fourfold, fossil remains from this period are much less studied, because living organisms at that time were predominantly small, lacked hard skeletons, and were poorly preserved. Phanerozoic deposits, especially continental ones, have been studied in great detail.

CRYPTOZOIC

The Cryptozoic (also referred to as the Precambrian) encompasses the oldest documented eras.

Archean Era (began over 3,500 million years ago).

It is characterized by intense volcanic activity and a lack of fossil remains. During this time, The formation of the Earth's crust continued (throughout the Archean, it was still characterized by high temperatures, ubiquitous mobility, and complex plastic deformations), and the cores of all ancient platforms were formed. Most scientists believe the atmosphere at that time was oxygen-free (free oxygen content did not exceed 1% of the modern level).

The primordial World Ocean was ten times smaller in volume than the modern one and was replenished through the degassing of the planet's deep interior. The ocean differed in chemical composition from the modern one while maintaining a comparable overall salinity; the salt content was formed by the dissolution of gases in water and the weathering of rocks.

It is believed that in the late Archean (no remains of any organisms are known in ancient strata), protobionts may have emerged at the bottom of shallow water bodies—the first primitive living organisms that were initially heterotrophs and fed on Organic compounds of the primordial soup accumulated during the planet's prebiological development. With the advent of more complex biological systems, some of them began using solar energy to synthesize new cellular material, leading to The Development of autotrophy (not all scientists agree with this sequence of events, as will be discussed in more detail later). At the same time, according to some paleontological findings, The Emergence of Photosynthesis dates back approximately 4 billion years ago, and the formation of heterotrophs even earlier, which pushes the appearance of life on our planet back almost to the moment of its formation as a celestial body.

Proterozoic Era (began about 2,500 million years ago).

It lasted longer than the others—nearly two billion years; it was characterized by extensive erosion processes, intense sedimentation, late volcanic activity, and repeated glaciations. The Proterozoic consists of two periods: the Karelian and the Riphean.

During the Karelian (about 2,500 million years ago), the cooled Earth's crust began to fracture, resulting in the Separation of platform and geosynclinal (depression) regions—ancient platforms were formed. The thermal regime of the Earth's crust and its mobility decreased compared to the Archean, and tectonic stabilization began. At this time, one of the most important aromorphoses in the evolution of life occurred: true plants appeared (blue-green Algae), which, alongside Bacteria and amoeboid organisms, were the dominant inhabitants of water bodies at that time.

During the Riphean (about 1,650 million years ago), several tectono-magmatic epochs occurred, during which the Pacific Ocean basin formed and the platforms of the Northern Hemisphere separated from the southern supercontinent of Gondwana. Two major events took place in the living world: the appearance of eukaryotes and Multicellular Organisms.

With the advent of eukaryotic forms, the need arose for the precise transmission of hereditary information, leading to the emergence of mitosis, Meiosis, and sexual reproduction. THE ORIGIN OF multicellularity contributed to the diversification of existing life forms, increasing ecosystem stability, and also led to the disappearance of immortality. With the emergence of free oxygen of photosynthetic origin, the restructuring of the atmosphere and hydrosphere began, and weathering and soil-formation processes intensified. The Oxidation of ammonia led to the formation of molecular nitrogen and caused a significant change in The chemical composition of the atmosphere. The oxidation of methane and carbon monoxide enriched seawater with carbonates, and sulfur compounds with sulfates (bringing the chemical composition close to the modern one).

It is believed that the Riphean was a time of extraordinary development for blue-green algae, as this period witnessed their remarkable systematic diversification. Later (at the boundary with the Vendian), a significant reduction in both the numbers and diversity of these forms occurred due to their inability to compete with eukaryotes. Unfortunately, discussing the Development of the animal world in more detail is still difficult due to a lack of factual material.

Vendian (about 650 million years ago).

Scientists have many debates regarding the Classification of this era (some consider it the final period of the Proterozoic). It was characterized by large land areas and high volcanic activity.

At the beginning of the Vendian, significant glaciation took place, preceding a major reorganization of the organic world (sea levels dropped sharply, and salinity increased). Subsequently, the Vendian marine transgression (the advance of the sea onto land) began, creating more favorable conditions for the development of organisms and triggering an evolutionary "explosion" of diverse forms of organic life.

The smooth Topography of the Earth's surface and high atmospheric carbon dioxide levels fostered a warm and quite humid climate across most of the planet. Barren landmasses were dominated by desert landscapes. Algae flourished in the plant kingdom, and Fungi (actinomycetes) began to emerge. Benthic multicellular algae experienced a remarkable expansion, accompanied by an Abundance of microscopic phytoplankton.

Unlike previous periods, animal life developed vigorously during this time, with most major groups appearing almost abruptly—a phenomenon likely driven by profound ecological and climatic upheavals. Despite a significant diversity of high-ranking taxonomic categories, most groups had not yet achieved substantial species richness.

Vendian fauna were predominantly soft-bodied (the fossil record lacks forms with well-developed skeletons), featuring the appearance of radiolarians, Sponges, Coelenterates, Annelids, unmineralized Mollusks, Echinoderms, and the earliest Arthropods. Scientists attribute the exceptional preservation of these soft-bodied creatures to the absence or negligible role of scavengers and other decomposers in the biosphere at that time. Consequently, nearly all major animal phyla were already established by the end of the Cryptozoic.

PHANEROZOIC

The Phanerozoic (derived from the Greek *phaneros*, meaning visible, manifest, or evident, and *zoe*, meaning life) is the eon of visible life, characterized by an abundant and well-preserved fossil record.

Paleozoic (570 million years ago). The most critical milestone of this era was the colonization of land by living organisms, which accelerated the overall pace of organic evolution. Supercontinents (Pangaea, Gondwana, Laurasia) and their tectonic transformations exerted a profound influence on this process.

Cambrian (570 million years ago). The period began with a major marine transgression (sea levels rose by 140 m), followed by a partial regression of water mid-period. There were no sharp temperature gradients between polar and equatorial regions at this time, with average temperatures reaching 20–28 °C.

The flora was marked by the proliferation of blue-green, calcareous, golden, and, to some extent, green algae. This period and the two that followed are widely regarded as the age of algae.

Vendian fauna gave way to animals possessing skeletons, shells, and protective carapaces. The acquisition of protective outer layers by various invertebrate taxa is attributed to their newly developed ability to mineralize (previously hindered by high concentrations of carbon dioxide, chlorine, and ammonia in the water, which dissolved mineral components). Trilobite-like organisms (a subtype of marine arthropods) experienced a remarkable diversification; archaeocyathids (sponge-like, reef-building animals with calcareous skeletons) appeared and subsequently went extinct; and foraminifers, sponges, corals, bivalves, gastropods, hyoliths (an extinct group of animals morphologically close to mollusks), and brachiopods with chitinous skeletons made their debut. According to some specialists, the earliest vertebrates also emerged during this period. While the Cambrian witnessed an explosive radiation of species, a significant number of these forms vanished just as quickly.

Ordovician (500 million years ago). Its first half was marked by one of the largest marine transgressions in the Proterozoic, ultimately submerging nearly half of modern continental landmasses. The increased relative area of the oceans led to a general humidification of Earth's climate. However, by the end of the period, sea levels dropped sharply, intensifying continental aridity, narrowing the tropical belt, generating numerous glaciers, and creating pronounced temperature contrasts between polar and equatorial zones.

The plant kingdom was dominated by blue-green, green, golden, euglenoid, and certain other algae. It is hypothesized that higher plants already emerged during this period—evidenced by the discovery of primitive rhyniophyte spores—though they did not yet play a major role in the biosphere.

In the animal kingdom, the most pivotal developments included the appearance of graptolites (hemichordates), sea urchins among echinoderms, cephalopod mollusks, numerous other invertebrate forms, and the first vertebrates—agnathans (*Hemicyclaspis*, *Poraspis*, etc.).

Silurian (440 million years ago). The climate largely mirrored that of the Ordovician, though average temperatures rose (reaching 18–22 °C by the end of the period), melting valley glaciers and fostering thermophilic biotic assemblages. Continents featured gently rolling topography, devoid of major mountain systems. Northern landmasses began to coalesce and drift southward.

The organic world underwent profound paleoecosystem restructuring. In marine environments, algae—particularly red and brown algae—proliferated extensively. It was during this epoch that higher plants finally colonized the land (initially forming rhyniophytes, followed later by lycophytes). *Cooksonia* (belonging to the rhyniophytes) is considered the oldest representative of this flora.

Among invertebrates, reef-building corals (tabulates and rugosans), bivalved crustaceans (ostracods), and massive marine arthropods up to 2 meters in length—eurypterids, or sea scorpions—reached their peak diversity. The first invertebrates ventured onto land (it is believed that landmasses were still devoid of vertebrate inhabitants at this time). A small number of early armored Fishes (*Pteraspis*) also appeared during this period.

Devon (405 million years ago). The expansion of land and shrinkage of ocean coverage continued intensely, marked by a notable reduction in epicontinental seas. Terrestrial development was accompanied by significant rifting and volcanic outpourings. The climate was predominantly tropical and equatorial. Global average temperatures reached 24 °C (peaking at 28 °C in the Middle Devonian), driven by the low-to-mid-latitude positioning of continents, the abundance of epicontinental marine basins, and high atmospheric carbon dioxide levels.

In the Early Devonian, zosterophylls (semi-aquatic plants) evolved from rhyniophytic ancestors, but starting in the Middle Devonian, herbaceous psilophyte vegetation began to give way to the first forests composed of lycophytes, horsetails, and ferns. The rapid spread of higher seed plants (seed ferns and progymnosperms—transitional forms between spore plants and gymnosperms) during the Devonian was likely facilitated by high atmospheric humidity and abundant rainfall, which bridged the environmental gap between aquatic habitats and coastal lowlands. The extraordinary development of plant cover, primarily in coastal regions, enriched the air with oxygen, which is believed to have practically reached modern levels by this time.

Overall, the Devonian is celebrated as the "Age of Fishes," during which armored, cartilaginous, and bony fishes coexisted. Cephalopods (nautiloids and ammonites) and armored fishes (*Pteraspis*, *Coccosteus*, *Pterichthyodes*) flourished widely, and lobe-finned fishes emerged, giving rise to the first tetrapods—stegocephalians (marking the vertebrate invasion of land). Terrestrial ecosystems saw the appearance of scorpions, mites, millipedes, collembolans, and dragonflies. Graptolites and ancient lophophorates began to decline, and by the end of the period, all jawless fish had vanished (the evolutionary trajectory of the cyclostome Lineage remains largely enigmatic).

Carboniferous, or the Coal Age (350 million years ago), is divided into two epochs: the Lower (Mississippian) and Upper (Pennsylvanian) Carboniferous. Early in the Carboniferous, the collision of Gondwana with northern landmasses forged Pangaea, which can be considered the defining tectonic event of the period. Two major cycles of marine transgression and regression occurred, and by the end of the Carboniferous, vast tracts of land were transformed into swamps. Continental reorganization triggered dramatic climatic shifts. Pronounced cooling and distinct climatic zonation emerged, resulting in continental ice sheets and mountain glaciation across Gondwana.

The Carboniferous is renowned as a period of exceptional plant proliferation. Forests were dominated by calamites (giant horsetails), lepidodendrons, sigillarians (arborescent lycophytes), and others. Seed plants developed actively—initially seed ferns and gymnosperm cordaites, followed by conifers toward the end of the period. By the close of the Carboniferous, severe cooling in the Southern Hemisphere caused dwarf shrubby and herbaceous forms to dominate the flora.

Among invertebrates, large foraminifers (fusulinids) became widespread. The earliest belemnites appeared, alongside numerous spiders, cockroaches, and flying insects (giant dragonflies, orthopterans, hemipterans). Cartilaginous and bony fishes became dominant in marine habitats, while stegocephalians achieved remarkable diversity on land. In the cooler Southern Hemisphere, the first primitive reptiles (cotylosaurs) emerged. This period witnessed the final disappearance of graptolites, tentaculites, armored fishes, and several other Organism groups.

Permian (285 million years ago). The period commenced with a major marine regression, triggering a prolonged arid epoch that persisted into the Triassic. Throughout the period, Pangaea stretched from the South Pole to the North Pole. Glaciation remained widespread in the southern half (Gondwana), leaving geological traces not only in Antarctica but also in Africa, the Indian subcontinent, Australia, and South America.

At the beginning of the period, the vegetation resembled that of the Carboniferous, though latitudinal zonation became more pronounced. Rapid climatic aridization drove the mass extinction of spore-bearing plants—especially in Laurasia, where they formed extensive coal deposits—and fueled the widespread expansion of gymnosperms, including conifers, cycads, and ginkgos.

Marine ecosystems saw the diversification of fusulinids, brachiopods, cephalopods, and sharks, while insects proliferated on land (including the advent of neuropterans, orthopterans, beetles, etc.). Reptiles (mesosaurs, batrachosaurs, therapsids, lepidosaurs, and others) dominated the southern regions, whereas stegocephalians prevailed in the north. By the end of the Permian, trilobites, eurypterids, and rhipidistians went extinct; formerly ubiquitous groups such as echinoderms, tabulates, and rugosans declined sharply; and stegocephalians were nearly wiped out. Numerous vital life forms that had defined the Paleozoic Era vanished completely.

Mesozoic (230 million years ago)—the Age of Reptiles. It is viewed as a transitional bridge between the Late Paleozoic biota, which dominated the early part of the era, and the Cenozoic biota, which began to take shape in the final period. Throughout this era, the supercontinent Pangaea steadily broke apart—first splitting into Gondwana and Laurasia, and subsequently fracturing into the modern continents. Reptiles reigned supreme as the apex forms across terrestrial, aquatic, and aerial realms.

Triassic (230 million years ago). The beginning of the Triassic was marked by a rather arid climate, driven by continental uplift, while its second half experienced marine transgression and the final breakup of Pangea into Gondwana and Laurasia. These large-scale tectonic processes were accompanied by intense volcanic activity. The climate gradually became more arid (cooler and drier).

The floristic COMPOSITION OF THE land, as well as marine and terrestrial faunas, continued to change. Seed ferns died out, while cycads, ginkgoes, bennettitales, and conifers underwent a widespread expansion among gymnosperms.

Rugose corals, bivalves, ammonites, and sea urchins achieved significant development. Paleozoic sharks and lobe-finned fish disappeared, giving way to the first plesiosaurs and ichthyosaurs. By the end of the period, the first teleost fish appeared. On the landmasses of both hemispheres, most ancient stegocephalians died out, marking the beginning of the age of reptiles (turtles, primitive dinosaurs, crocodiles, and pterosaurs emerged). The first true mammals appeared (small, unspecialized prototherians and marsupials). Insect fauna also diversified significantly: Diptera, Hymenoptera, Lepidoptera, and several other groups spread widely.

Jurassic (195 million years ago). During the Jurassic, Gondwana—which had existed as a single continent for over 350 million years—began to split into four major fragments: Hindustan, Antarctica with Australia, South America, and Africa with Arabia. These exact processes led to the Formation of the Indian Ocean, and later, following the separation of Africa and South America, the creation of the Atlantic Ocean began. Two phases of Alpine mountain building became apparent, and North America started to detach from Europe (significant parts of these continents were covered by shallow seas).

The main event in the plant kingdom during this period was the appearance of angiosperms (according to modern concepts, the emergence of this group should be dated much earlier). However, the dominant forms remained various ferns and gymnosperms (cycads, ginkgoes, bennettitales, conifers, etc.).

In the marine fauna, the species composition of ammonites, corals, and crustaceans shifted. Belemnites, echinoderms, ichthyosaurs, and plesiosaurs became widespread. Reptiles flourished in aquatic and aerial environments, as well as on land. The most famous group was the dinosaurs (about 600 species are now known), which were divided into two orders: Saurischia and Ornithischia. Some of them, predominantly herbivores, reached enormous sizes—up to 30 m in length and weighing over 30 tons. Pterodactyls and rhamphorhynchs ruled the skies, some with wingspans of up to 15 m. Mammals remained virtually unchanged in their Organization, and primitive birds, or lizard-tailed birds (Archaeopteryx), appeared.

Cretaceous (137 million years ago). Throughout this period, Gondwana finally broke apart, and the southern part of the Atlantic Ocean formed. The separation of North America and Europe began. These events led to the reduction of the Tethys and Pacific oceans and the expansion of the Indian and Atlantic oceans. Active mountain building took place (the Andes, Alps, Himalayas, and other mountain systems were formed).

In the plant kingdom, angiosperms (Magnoliaceae, Lauraceae, Platanaceae, certain Fabaceae, Fagaceae, Salicaceae, Moraceae, and among monocots—palms, certain grasses, and a significant number of other forms) began to displace gymnosperms. By the end of the Cretaceous, bennettitales, caytoniales, most ginkgophytes, and other groups disappeared.

Reptiles were still at their peak: it was during this period that modern forms such as snakes, lizards, and true turtles appeared. Towards the end of the period, true birds (Hesperornithes, Ichthyornithes, and later toothless forms) and placental mammals (insectivores and early condylarths) emerged. A significant portion of groups (ammonites, belemnites, rudist bivalves, and the vast majority of dinosaurs) began to die out almost without a trace (the so-called Great Mesozoic Extinction, during which nearly 2/3 of biotic species vanished).

Cenozoic (nearly 66 million years ago). The modern configuration of the continents formed during this era. In aquatic environments, true teleost fish replaced dinosaurs; in the air, birds; and on land, mammals.

Tertiary period (two epochs were later distinguished, which are sometimes elevated to the rank of periods).

Paleogene (66 million years ago). This epoch comprises three epochs/ages (Paleocene, Eocene, and Oligocene). Overall, the period is characterized by intensive mountain-building processes (the main ridges of many Eurasian mountain systems formed), the rift valleys of East Africa emerged, and the formation of the North Atlantic was completed.

The plant kingdom maintained the developmental trends that began in the late Mesozoic, with flowering plants becoming dominant. A crucial development was the establishment of a floral zoning corresponding to climatic features. Due to a significant deterioration of the climate in the Oligocene (marked cooling and aridization), forest areas gradually decreased, and open steppe-type landscapes formed. Evergreen forests (myrtles, laurels, ficuses, certain palms, etc.) began to be replaced by deciduous ones.

The events of this epoch led to the final extinction of representatives from many systematic groups: ammonites and belemnites, most Mesozoic reptiles, and snake-like and armored fish disappeared without a trace. The active evolution of bivalves and gastropods, true teleost fish, typical birds, and mammals began; insects (especially Hymenoptera and Lepidoptera) underwent rapid evolution, which drove the co-evolution of pollinating insects and flowering plants. The Paleocene saw the flourishing of primitive mammals: creodonts (carnivores), condylarths (ungulates), insectivores, proboscideans, semi-aquatic forms, and others. In the Eocene, they were joined by more modern forms—horses (initially four-toed, then three-toed), pigs, lemurs, and monkeys. In the Oligocene, creodonts and condylarths died out, replaced by forms such as felines, canines, beavers, rhinoceroses, tapirs, rodents, and other mammals.

Neogene (25 million years ago). This epoch consists of the Miocene and Pliocene. Its defining feature was the most severe continental drought throughout the entire Phanerozoic. In addition, the formation of the Alpine mountain systems was completed, Antarctica became ice-covered, glaciers formed in mountainous regions, and a general cooling of the Earth took place. In response to these changes, forest areas shrank, and the flora of Europe and North America became impoverished.

In the Miocene, Paleogene vegetation (palms, myrtles, laurels, and other evergreen species) was replaced by broad-leaved forests, and steppes formed. In the Pliocene, cooling and increased continentality proceeded even more vigorously, resulting in the emergence of the taiga biome. By the end of the epoch, all warmth- and moisture-demanding forms (Liquidambar, sequoia, chestnut, walnut, hornbeam) virtually vanished from the temperate zone, and savanna-steppes gave way to dry steppes. By The final stage of the Pliocene, a plant cover close to the modern one was established.

It was during this time that the fauna of Eurasia was at its most diverse: insectivores, bats, numerous small rodents, mastodons, deinotheres, swamp rhinoceroses, true antelopes, and others. Due to the significant expansion of steppes, the Hipparion fauna formed (ostriches, giraffes, antelopes, rhinoceroses, camels, pigs, saber-toothed cats, and Hipparion—a three-toed horse). North America's species diversity was poorer, whereas South America saw the development of marsupials, peculiar ungulates, rodents, and platyrrhine monkeys. Australia remained isolated, allowing for the further independent development of marsupials (several independent centers of mammalian development existed). A distinctive feature of the period was the evolution of anthropoid apes.

Quaternary period, or Anthropogene (about 2 million years ago). Characterized by multiple cooling and warming cycles. During cold phases, continental glaciers formed, causing the sea level to drop by 100–150 m (global sea-level fluctuations). All of this triggered further shifts in flora and fauna across virtually the entire planet. The main direction of change persisted from the Tertiary period—southern warmth-loving forest flora and fauna were replaced by cold-tolerant steppe forms. Due to the lack of escape routes during glaciations, most warmth-loving plant taxa disappeared, giving the flora a practically modern appearance.

The glacial period (Pleistocene) was characterized by significant temperature and overall climate fluctuations, which drove corresponding animal and plant Migrations as well as the emergence of new, specific forms (mammoths and woolly rhinoceroses appeared during the Würm glaciation). The fauna of the northern continents became greatly impoverished and acquired a modern aspect. This era encompasses anthropogenesis, culminating in the emergence of Homo sapiens sapiens (modern humans). The last 10–11 thousand years, the so-called post-glacial period (Holocene), have been marked by active human impact on the environment (especially over the last 400 years).