WHEN DID HUMANS APPEARS?

Yesterday, Today, and Tomorrow

When did humans appear? What is it that makes us different from the rest of the animals? In what way did language develop? Why is it so important to have deciphered the sequence of the human genome? This book offers answers to these and many other questions about the mysteries and marvels of human evolution. Scientists maintain that modern humans originated in Africa because that is where they have found the oldest bones. In addition, genetics has just arrived at the same conclusion, since the DNA studies have confirmed that all humans are related to the African hunter-gatherers who lived some 150 million years ago.

Studying the fossils, the experts also found that human skulls from two million years ago already show the development of two specific protuberances that in the present-day brain control speech, the capability that perhaps was as important for early humans as the ability to sharpen a rock or throw a spear.

Today thanks to science it is possible to affirm that the brain has changed drastically in the evolutionary course of the species, reaching a greater complexity in humans. This has facilitated, among other things, the capacity to store information and the flexibility in behavior that makes a human an incredibly complex individual. The purpose of this book is to tell you and show you in marvelous images many of the answers that people have found throughout history, through their successes, failures, and new questions. These new questions have served to shape the world in which we live, a world whose scientific, technological, artistic, and industrial development surprises and at times frightens us.

History is full of leaps. For thousands of years nothing may happen, until all of a sudden some new turn or discovery gives an impulse to humankind. For example, with the domestication of animals and the cultivation of plants, a profound societal revolution occurred. This period of prehistory, called the Neolithic, which dates to 10 million years ago, opened the way for the development of civilization. With the possibility of obtaining food without moving from place to place, the first villages were established and produced great demographic growth.

In the decades to come, the application of genetic therapy will allow, among other things, the cure of genetic disorders caused by defective genes. In addition, the alternative of knowing beforehand what diseases a person could develop will be extremely valuable in the field of health, because we will be able to choose examinations and treatments according to individual needs. Another very promising area of medical research involves the use of stem cells that have the unique capacity to be used at some future date to regenerate organs or damaged tissues. Do not wait any longer.

Myths and Scientific Evidence

The evolution of species cannot be considered an isolated event in itself but rather the result of a complex and constant interaction among different elements. It represents not simply an unlimited number of genetic mutations but also changes in the environment, fluctuations in sea level, varying contributions of nutrients, and possibly factors such as the reversal of the Earth's magnetic field or the impact of large meteorites on the Earth's surface.

Various Beliefs

Before the emergence of scientific theories, most people in the world had their own versions of the origin of the world and of humankind expressed primarily in the form of myths. Many of them have reached us through the teachings of different religions. In many cases, the origin of the world and of humankind relates to one or several creator gods or demigods; in other cases, there is no beginning and no end. With regard to the origin of the human race (the word “human” shares the same root as the Latin word humus, meaning “earth”), there is a Central African legend that links humans to monkeys.

The Matter of Creation

India is a multicultural, agricultural society where much of its thousand-year-old rituals still exist. However, its sacred texts were written at very different times, from 1,000 BC (the Rigveda) to the 16th century AD (the Puranas), and they offer different versions of the origin of humankind. One of them even tells of a primal man (Purusha) from whom gods originated and from whose body parts the different castes arose. In this culture, social classes are strongly differentiated.

BRAHMA, THE CREATOR
Another version states that the first human emerged directly from the god Brahma.

HERMAPHRODITE
According to more recent texts (from the 15th century), the first person Brahma created was called Manu, and he was a hermaphrodite. The story goes that as a result of his dual sexual condition, he had a number of children, both males and females.

Disobedient

Judaism, Islam, and the various forms of Christianity adhere to the book of Genesis in the Bible, according to which the world was created by God in seven days. According to this account, the first human was created on the sixth day “in the image and likeness” of the Creator. The intention was for this new creature to rule over nature. The first woman, Eve, emerged from one of Adam's ribs. Because they disobeyed the Creator by eating one of the forbidden fruits, Adam and Eve were banished from Paradise. Condemned to work the soil and for woman to suffer during childbirth, they had three sons, from whom the human race descended.

EDEN
The biblical story locates the earthly Paradise in Mesopotamia. In Paradise, all the living species lived, and humans had only to take what they needed.

THE TWO SEXES
Although Genesis is somewhat contradictory on this point, the dominant version states that God created Eve from one of Adam's ribs while he slept.

HUMAN SHAPES
Christianity represented the Creator and the angels in human form, but Judaism and Islam did not assign a human likeness to their God.

FORBIDDEN FRUIT
According to the biblical account, Adam and Eve ate the fruit of the Tree of Knowledge of Good and Evil.

The Divine Breath

The story explains that God gave life to inert matter through either breath. In many other cultures, life is also identified with the breath of the creator of the world. In Egyptian mythology, for example, the breath of the god Ra, “The Limitless God,” transforms into air (Shu), which is the indispensable element of life.

Africa: How Monkeys Became Human

In Africa, the continent that is today believed to be the cradle of the human species, there are several myths that account for the origin of mankind. One of these actually interweaves it with the origin of the monkey. It tells how the creator god Muluku made two holes in the Earth from where the first woman and the first man sprouted and how he taught them the art of agriculture, but they neglected it and the Earth dried up. As punishment, Muluku banished them to the rainforest and gave them monkey tails, and he removed the tails from monkeys and ordered them to be “human.”

Evolution Is a Matter of Time

Toward the 18th century, scientific progress demanded a different explanation of the myth of the origin of the world and of life. Even before Darwin, the work of naturalists and the discovery of fossils pointed to the fact that time, measured not in years but in millennia, runs its course, allowing each species to become what it is.

Genetic mutations occur through the generations, and interaction with the environment determines that the most suitable traits will be transmitted (natural selection) and that a population will evolve in relationship to its ancestors. The idea is not related to “improvement” but rather to change as the origin of diversity, to the ramifications of evolutionary lines tracked through paleontological or genetic studies.

A Common History

Animals that look very different may be built according to the same basic body design. For example, dogs, whales, and human beings are mammals. All have the same skeletal design with a spinal column and two pairs of limbs connected to it. This suggests that they all share a common ancestor. In mammals, the bones of the limbs are the same even if they are morphologically different from one another.

Evolutionary Processes

In addition to natural selection, the famous theory developed by Charles Darwin in the 19th century, there are other evolutionary processes at work at the microevolutionary scale, such as mutations, genetic flow (i.e., migration), and genetic drift. However, for evolutionary processes to take place, there must be genetic variation — i.e., modifications to the proportion of certain genes (alleles) within a given population over time. These genetic differences can be passed on to subsequent generations, thereby perpetuating the evolutionary process.

Natural Selection

This is one of the basic mechanisms of evolution. It is the process of species survival and adaptation to changes in the environment, and it involves shedding some traits and strengthening others. This revolutionary transformation takes place when individuals with certain traits have a survival or reproduction rate higher than that of other individuals within the same population, thus passing along these genetic traits to their descendants.

Mutation
involves the modification of the sequences of genetic material found in DNA. When a cell divides, it produces a copy of its DNA; however, this copy is sometimes imperfect. This change can occur spontaneously, such as from an error in DNA replication (meiosis) or through exposure to radiation or chemical substances.

Genetic Flow
The transfer of genes from one population to another occurs particularly when two populations share alleles (different versions of genes). For example, when a population of brown beetles mixes with a population of green beetles, there might be a higher frequency of brown beetle genes in the green beetles. This also occurs when new alleles combine as a result of mixing, as when Europeans mixed with Native Americans.

Genetic Drift
A gradual change in the genetic makeup of a population that is not linked to the environment. Unlike natural selection, this is a random process that does not generate adaptations. Genetic drift is present in small populations in which each individual carries within itself a large portion of the genetic pool, especially when a new colony is established (the founding effect), or when a high number of individuals die and the population rebuilds from a smaller genetic pool than before (the bottleneck effect).

To Live or Die

Coevolution is a concept used by scientists to describe the evolutionary process from a group perspective, because no single species has done it in isolation. On the contrary, different levels and types of relationships were established through time between species, exerting changing pressures on their respective evolutionary paths. Natural selection and adaptation, both processes that every species has undergone to the present, depend on these relationships.

Types of Relationships

If the evolution of each species were an isolated event, neither the relationships nor the adaptations that together generate coevolution would exist. In fact, in the struggle for survival, some species react to the evolutionary changes of other species. In the case of a predator, if its prey were to become faster, the hunt would become more difficult and a demographic imbalance would develop in favor of the prey. Therefore, the speed of each depends on the mutual pressure predator and prey exert on each other. In nature, different types of relationships exist that are not always clear or easily discernible given the complexity they can acquire through the process of coevolution. These range from noninteraction to predation, from cooperation to competition and even parasitism.

Commensalism
is a relationship between two species of organisms in which one benefits and the other is neither harmed nor helped. There are several types of commensalism: phoresy, when one species attaches itself to another for transportation; inquilinism, when one species is housed inside another; and metabiosis, such as when the hermit crab lives inside the shell of a dead snail.

Mutualism
is a type of interspecific relationship in which both species derive benefit. It might seem as if this is an agreement between parties, but it is actually the result of a long and complicated process of evolution and adaptation. There are numerous examples of mutualism, although the most famous is the cattle egrets of Africa (Bubulcus ibis), which feed on the parasites of large herbivores such as the buffalo and the gnu. To the extent that the egrets obtain their food, the herbivores are rid of parasites.

Parasitism
is defined as an asymmetric relationship in which only one of the organisms (the parasite) derives benefit. It is an extreme case of predation that entails such fundamental adaptations where the parasite, which enters by various means, might even live inside its host. Such is the case of the African buffalo, which can have a worm called Elaeophora poeli lodged in its aorta.

Competition
takes place when two or more organisms obtain their resources from a limited source. This is a relationship thathas one of the strongest impacts on natural selection and the evolutionary process. There are two types of competition. One occurs through interference, which is when an action limits another species' access to a resource—for example, when the roots of a plant prevent another plant from reaching nutrients. The other type of competition is through exploitation, typical among predators such as lions and cheetahs that prey on the same species. In this second type, the principle of competitive exclusion is also at play, since each species tends to eliminate its competition.

Predation
is the interspecies relationship in which one species hunts and feeds on another. It is important to understand that each party exerts pressure on and regulates the other. There are specific instances of predation in which the hunter impacts only one type of prey or those in which it feeds on different species. The degree of adaptation depends on this distinction. The lion, the zebra, and the kudu form an example of the latter case.

The Critical Point

One of the big issues posed by the theory of evolution is how a new species arises. This presumes that a population becomes separated from other individuals within its group (when, for example, it lives under conditions different from those of its parents) and ceases to interact with them. Through the generations, the isolated individuals will experience genetic mutations that give rise to phenotypic changes completely different from those experienced by the original population to which they once belonged, and they develop traits so distinct that they become a new species. From an evolutionary perspective, this is how one can understand the constant emergence of new lineages and the growing diversity of living beings.

The origin of new species

Individuals of the same species look alike and breed among themselves, but not with those of other species. In speciation, two or more species arise from a single species (cladogenesis), or several fertile individuals arise from the crossbreeding of two different species (hybridization), although the latter is much less frequent in nature. Cladogenesis can arise out of geographical isolation or simply through a lack of genetic flow between groups of individuals of the same species, even if they are present in the same territory.

Selection
In spite of their differences, dogs are so similar to each other that they can breed with each other. They are in the same species. But selective breeding is a good example of how differentiation is favored, except that in nature it takes a longer time to do this. Selection can be disruptive, when two populations separate and become differentiated; directional, when the dominant traits of a population change; or stabilizing, when variations diminish and individuals become more similar to each other.

In "Britannica Illustrated Science Library" Vol. V, Evolution and Genetics,  Project Manager Fabián Cassan, Executive Editor Michael Levy, Encyclopædia Britannica, Inc. excerpts p.7-17. Adapted and illustrated to be posted by Leopoldo Costa.