Evolution: animal life

According to the American Museum of Natural History, the Big Bang occurred nearly 14 billion years in the past. The Big Bang was a high-temperature, high-density explosion that marked the beginning of the universe. The Sun and the Earth formed about 4.6 billion years ago. Four billion years ago, the newly created Sun shone with only 70 percent of its current strength. The Earth’s atmosphere had no oxygen. Bacteria, viruses, plants, and animals were not in existence. Subsequently, chemical processes are believed to have created oxygen. What materialized next was an organic “soup” of microbial life in the form of the simplest cells without a nucleus, called prokaryotes. These prokaryotes included bacteria and developed out of the primordial ooze. These bacteria were the only living organisms on the Earth for about two billion years. After that time, about 1.5 billion years ago, more complex cells with nuclei, eukaryotes, appeared. Before eukaryotes, bacteria were the only existing animal organisms on the planet.

Biological Classifications

Living organisms on the Earth are classified under a taxonomy, or a biological system of classification, first devised by Carolus Linnaeus (1707–1778). The primary groupings are called the domain, kingdom, phylum/division, class, order, family, genus, species, and subspecies. The most encompassing ranks begin with the domain. This is a general grouping and contains the largest number of organisms. There are only three domains and five kingdoms. A phylum is most used in reference to organisms in the Animal kingdom. Plants, bacteria, and protists are normally classified in a division rather than a phylum. Animals in a phylum have unique characteristics that make them different in some manner than other animals in a different phylum. The most basic and specific groupings are species and subspecies. The Earth contains approximately eight million species.

The Beginnings of Animal Life

Sometime between 1.3–2.6 billion years ago the botanical and the zoological, diverged, and millions of animals came into being. Animals are multi-celled and equipped with specialized body parts. They are distinguished from plants primarily by their methods of feeding, locomotion, and reproduction. Most of the phyla in the Animal Kingdom appeared during the Cambrian Era, an immense span of geological time that began about 541 million years ago. In a relatively short span of ten million years, all the animal phyla currently known came into being, perhaps encouraged by an increase in oxygen in the seas, where animal life began.

Eventually, animals developed a nervous system that enabled them to control their movements more appropriately, as well as sense organs to help them find suitable food. At the margins, however, the dichotomy between the botanical and the animal world has remained ambiguous, since there are many microorganisms that defy clearcut classification. At times, these difficult cases are known as Protista, or Protists.

In all forms of life, including the animal kingdom, no phylum has been produced by a single evolutionary event. Nor have different animal orders appeared as a result of sudden evolutionary changes. Rather, all have come about, whether in gradual or punctuated manner, by the cumulative effect of small steps in different directions. This, at least, is the theory posited by Charles Darwin’s explanation of evolution.

The timeline of the animal kingdom is closely connected with this evolutionary chronology. Darwin’s Theory of the Origin of Species through Natural Selection has been almost universally accepted in the scientific community. It has, at times, been opposed on Biblical creationist grounds. Natural Selection suggests that animals, like plants, have changed since the beginning of life on Earth and are still evolving. Also, there was no sudden creation of all species. Instead, over long periods of time, new species have evolved from isolated populations of existing species. These came to occupy new niches. Thus, all current species are changed descendants of others that existed previously. If there are fewer apparent links between phyla than between families further down the classification ladder, the reason is that phyla have had a longer history, and so have experienced more opportunities for the elimination of intermediate forms. From an evolutionary viewpoint, the difference between species down an evolutionary line is even more recent than those between families and so on. For species to have survived in their environments—with simultaneous changes in ecology, climate, and flora—many animal forms are now more complex and efficient than their ancestors used to be.

Genes and Evolution

Life began in the oceans. For animals to live in freshwater, and later on land, many adaptations had to transpire. According to what has been termed Darwin’s Theory of the Survival of the Fittest, the impetus for sea animals to migrate onto land came from competition, Fauna that managed to adapt were the ones that underwent waves of evolution and transformed into new kinds of animals.

It was only in the twentieth century that the discovery was made that the characteristics of a species are passed on from parents to offspring by genes. Genes provide cells with particular features, such as webbed feet. With such characteristics inherited by offspring from parents, there is a resemblance among generations. However, at times a parent may produce quite a different offspring because of genetic change. The young, in turn, may replicate this difference in their own descendants in a process known as mutation. Mutations may occur spontaneously for unknown reasons, but also from known causes, such as exposure to radiation. At times, mutations may be useful in allowing the species to adapt better to its environment. As an example darker moths have a better chance of survival in a forest than lighter ones because the latter are more visible to their predators than the former. Other mutations may be harmful, such as larger size that slows down a species, making its flight from danger more difficult.

Animals may remain the same, evolve, or become completely extinct within a time frame of 1 to 10 million years. This occurred to the dinosaurs about 60 million years ago, after being dominant for 350 million years.

Adaptation to Environment

Natural selection leading to a new species may be accelerated when members of the original species move to a new environment, whether voluntarily or driven by the elements. Separated populations may develop different traits as they adapt to their new condition. Eventually, they will become sufficiently different to be unable to produce offspring with members of their original population. This process has repeated itself many times, over millions of years, and accounts for the large diversity in the animal world, not to mention the additional diversity consequent on artificial breeding by humans, widely observed among domesticated animals such as horses, cattle, and dogs.

Animals occupied new environments as species living in water moved to the land or later to the air. Thus, the step from fish to amphibian was essentially one from living in water for the whole life to living on land for the adult stage of the life cycle. The step from amphibian to reptile was one of increasingly proficient adaptation to land life at all stages of the life cycle.

Birds and mammals evolved in different directions from the reptiles, the first in adaptation to an arboreal and finally a flying life and the second as a further advance in the maintenance of an even and high body temperature—homeothermy—by combining an insulating external layer such as hair with a variety of physiological thermostats.

Events in geology, climate, and flora also determined the geographic distribution of species. Thus, marsupials are currently found almost entirely in Australasia and South America. The tiger exists only in India and Southeast Asia. The lion is restricted mainly to Africa. This pattern reflects the way in which these groups have evolved in relation to the physical world.

New animal groups evolve into many different forms, especially when they become dominant. For instance, when mammals came to occupy the dominant position, some became meat-eaters while others became vegetarians; some became smaller while others became larger; some became runners while others ended up as burrowers or flyers; still others returned to the water. This trend allowed the descendants of the original type to exploit a much greater range of environments and resources. Essentially, those species whose sense organs or brain morphology and functions improved the most ended up being dominant—primates in general and humans in particular.

Time Frame

The exact time of the origins of animals during earth’s evolutionary history is not known because the early species were soft-bodied, at first single-celled and later multicellular life forms, that did not fossilize well. Fossils are the best material evidence of archaic times. Fossils do not appear earlier than 650 to 500 million years ago, not only because the animal life of the time was inappropriate for fossilization, but also because continued crustal shifts in the ensuing eons disturbed the very early rock formations. Accordingly, fossil evidence is unavailable for the entire early history of animals, which must consequently remain speculative. Current taxonomic interrelationships suggest the early history, and taxonomic diagrams may be regarded as presumptive evolutionary diagrams as well.

Even though over a million different species of animals have been identified, it is suspected that a similar number remain to be discovered or became extinct before such identification could be made. In the United States alone, approximately forty species of birds, and about thirty-five species of mammals, and twenty-five other species have become extinct in the last two hundred years. This resulted from human activities such as the destruction of animal habitats through urbanization, the clearing of land for agricultural purposes, and pollution. Other contributors have been the introduction of new species from other parts of the world which turned out to be predatory to domestic specimens, hunting, and human population growth. It is widely predicted that climatic change triggered by greenhouse gases will continue, even enhance, this process, thereby endangering more animal species. Whatever the future, however, evolutionary biologists estimate that 99 percent of all species that have ever lived on earth are now extinct.

Despite these and other caveats, here is a very approximate timetable of the evolution of animal life:

Translated in terms of a single year:

January 1: Big Bang

March 22: Bacteria, the first living animals

November 9: Invertebrates

November 22: Vertebrates

December 16: Mammals

December 28: Primates, the highest order of mammals

December 31, a few minutes before midnight: Modern man, the dominant primate

Evolution of Existing and Extinct Human Species and Australopithecines

The root of the hominid evolutionary tree is still imperfectly known. The earliest australopithecine species, Australopithecus anamensis, is believed to be over four million years old, by which time that branch had diverged from African apelike ancestors. This species was followed by the Australopithecus afarensis nearly 3.5 million years ago. Much later came Homo habilis, called “skillful man” since they could presumably produce primitive tools, some two million years ago. They were followed by Homo erectus, “upright man,” about one million years ago. Finally, Homo sapiens, “knowing man,” emerged about 200,000 years ago. In the meantime, the australopithecine branch, after evolving through a number of intermediate species such as A. africanus, A. aethiopicus, and A. robustus, died out about one million years ago. To date, the earliest unearthed human-like primate fossil, that of Lucy, a three-foot-tall female discovered in Ethiopia, is about 3.2 million years old. The earliest fossil of an upright walker was discovered in the same area of Ethiopia in 2015 and is approximately 3 million years old. The oldest known fossils ever discovered, however, are those of bacteria and are in fact microscopic fossils of cells and bacteria "living" together in what was then an oxygen-free world over 3.4 billion years ago.

Modern humans are believed to have radiated out of Africa into Asia and Europe. Subsequently, cultural evolution became more prominent than biological evolution, but as modern humans evolved over the last four million years to their current condition, they developed manipulative skills, bipedalism, a change from specialized to omnivorous feeding habits, and especially, a threefold increase in cranial capacity from H. afarensis to H. neanderthalensis, together with behavior appropriate to the control of the environment.

Although humans are not the only animals capable of conceptual thought, they have refined and extended that ability until it has become their hallmark. Thus, thanks to the symbolic language of Homo sapiens, modern humans make possible the accumulation of experience from one generation to the next. Such cultural evolution is possessed by few, if any other animal species. It is for this reason that humans, more than other animals, have found ways to mold and change their environment according to need rather than in response to environmental demands. Because of this ability and humans’ control of technology, the species has more say about their biological future than any other.

Principal Terms

gene: the basic unit of heredity

gene flow: the movement of genes from one population to another

genetic drift: change in gene frequencies in a population owing to chance

interbreeding: the mating of closely related individuals, which tends to increase the appearance of recessive genes

migration: the movement of individuals, resulting in gene flow, changing the proportions of genotypes in a population

mutation: alteration in the physical structure of the DNA, resulting in a genetic change that can be inherited

natural selection: the process of differential reproduction in which some phenotypes are better suited to life in the existing environment and thus are more likely to survive

speciation: the formation of new species as a result of geographic, physiological, anatomical, or behavioral factors

species: the basic category of biological classification representing a group of potentially or actually interbreeding natural populations which are reproductively isolated from other such groups

taxon (pl. taxa): group of related organisms at one of several levels such as the family Canidae, the genus Canis, or the species Canis lupus

Bibliography

Carroll, Sean B., Jennifer K. Grenier, and Scott D. Weatherbee. From DNA to Diversity: Molecular Genetics and the Evolution of Animal Design. Malden: Blackwell Science, 2001. Print.

Conway-Morris, Simon. The Crucible of Creation: The Burgess Shale and the Rise of Animals. Los Angeles: Getty Center, 1999. Print.

Ghosh, Pallab. "'First Human' Discovered in Ethiopia." BBD. BBC, 4 Mar. 2015. Web. 31 Dec. 2015.

Golding, Brian. "Evolution: When was Life's First Branch Point?" Current Biology, June 1996, www.cell.com/current-biology/fulltext/S0960-9822(09)00448-5?. Accessed 2 July 2023.

"How Did the Universe Begin?" The American Museum of Natural History,www.amnh.org/explore/ology/astronomy/how-did-the-universe-begin#. Accessed 2 July 2023.

Lavers, Chris. Why Elephants Have Big Ears: Understanding Patterns of Life on Earth. New York: St. Martin’s, 2001. Print.

Long, John A. The Rise of Fishes: Five Hundred Million Years of Evolution. Baltimore: Johns Hopkins UP, 1996. Print.

"Oldest Fossils on Earth Discovered." Science Daily. Science Daily, 22 Aug. 2011. Web. 31 Dec. 2015.

"Phylum" Biology Online, www.biologyonline.com/dictionary/phylum. Accessed 2 July 2023.