EVOLUTION OF MAN: STAGES AND THEIR CHARACTERISTICS - BIOLOGY - 2025

The evolution of man, in biology, is one of the most exciting - and controversial - topics that exist in evolutionary biology, since it explains the origin of our own species; Homo sapiens.

One of the innate characteristics of human beings is curiosity about their origin. For this reason, the first edition of the work The Origin of Species was sold out on the first day of its publication.

Source: AquilaGib, from Wikimedia Commons

Although the masterpiece of the British naturalist, Charles Darwin, does not directly address the problem, he does so in his book published in 1871, "The Origin of Man."

The fossil record is one of the most useful tools for describing the process. Although imperfect, the hominid remains allow us to trace an evolutionary trajectory of the group, from the first australopithecines to modern humans.

Who is the man?

Before developing ideas about human evolution, it is necessary to understand who man is and how he relates - in terms of his phylogeny - to the rest of today's apes.

Humans are designated by the species Homo sapiens and are part of the primate taxon Catarrhini. This large group includes the Old World monkeys and the Hominoidea.

The hominoids include the genus Hylobates, popularly known as gibbon, which lives in the southeast region of Asia, and the Hominids. This last group includes the genera: Pongo, Gorilla, Pan troglodytes, Pan paniscus and Homo.

The first species, like the gibbon, lives in Asia, while the following species are native to Africa.

Currently, humans are considered to be grouped with the rest of the apes in Hominoidea. Since these share with the apes a series of derived characters, formally known as synapomorphies.

Synapomorphies

At the beginning of the development of modern systematics, the close relationship between humans and African great apes has been evident, mainly because of the synapomorphies between the two groups.

These shared derived characteristics allow the hominoids to be differentiated from the rest of the Catarrhini members, indicating that the homonoids descend from a common ancestor.

Among the most prominent we can mention: relatively large brains, mostly elongated skulls, robust and slightly shortened canines, absence of tail, upright position, flexibility in the joints, increase in the ovaries and mammary glands, among others.

Group relationships go beyond morphology. These investigations date back to 1904, when George Nutall used antibodies to show that the serum from chimpanzees was capable of reacting with those from humans - followed by that of gorillas, orangutans and monkeys.

Similarly, analyzes carried out at the molecular level using much more current technologies help to corroborate the morphological data.

How old are primates?

Paleontological evidence allows us to locate ourselves in the following time frame, in relation to the evolution of primates: protoprimates date from the Paleocene, later in the Eocene we find the first prosimians, at the beginning of the Oligocene we find the first monkeys.

The first apes emerged in the early Miocene, and the first hominids made their appearance at the end of this period, about 5.3 million years ago.

Stages in the fossil record: from pre-australopithecines to

According to estimates, humans and chimpanzees shared a common ancestor about 5 million years ago. What implications does this fact have? That probably the characteristics and behaviors that we share with this group of apes, we both inherited them from our common ancestor.

Note that we are not claiming that we are direct descendants of current chimpanzees. In evolutionary biology - contrary to popular belief - we must not assume that we come from any current form, since that is not the way evolutionary processes work.

We can trace our evolution thanks to the different fossil forms found after the divergence of our lineage with the chimpanzee.

Although the fossil record is not perfect - and does not come close to being considered "complete" - it has served as a small window into the past, allowing us to admire the forms of our ancestors.

We will start by describing each of the oldest fossils, following mostly the classification and names proposed by Johanson et al. 1996, and used by Freeman & Herron:

Sahelanthropus tchadensis

The first fossil we will mention is Sahelanthropus tchadensis. The remains of this individual were found in the Djurab desert, between 2001 and 2002. He lived about 7 million years ago.

The name of the fossil derives from Sahel, the region where the specimen was discovered. Likewise, the epithet refers to Chad, the country where the fossils were found.

Cranial and post-cranial remains of this species have been found (including a femur, which sparked a controversy that involved the Natural History Museum in Paris investigating them) of about 6 individuals.

The skull is small, the cranial crest is absent, and its general appearance is quite simian. The brain volume would be about 350 square cm, similar to the capacity of modern chimpanzees.

Experts have concluded that the organism can inhabit areas similar to swamps.

Orrorin tugenensis

This fossil corresponds to the first hominid with bipedal locomotion. It dates from about 6.2 to 5.8 million years, approximately. His remains are originally from Kenya and were found by a group of French and English paleontologists.

The teething of fossils allows certain predictions to be made about their eating habits and diet. The molars were conspicuous, while the canines were relatively small. It is presumed that their diet consisted of fruits.

It is also suspected that they resorted to herbivory, and that they added protein from insects.

Through the study of morphology, it is assumed that this genus is a direct descendant of Sahelanthropues tchadiensis and the ancestor of the next fossil that we will describe: Ardipithecus.

Ardipithecus ramidus

Tiia Monto, from Wikimedia Commons

Popularly known as "Ardi", A. ramidus dates back to about 4.4 million years and was found in Ethiopia. It is suspected that this organism could inhabit wooded ecosystems with humid climates.

Compared to modern humans, they were small individuals - they did not exceed 1.50 cm. Its cranial box exhibited a much smaller volume, of about 350 square cm.

Like Orrorin tugenensis, Ardi had a frugivorous or omnivorous diet, quite similar to that of current chimpanzees.

Australopithecines

Austrolopithecines are usually classified into two types depending on their appearance: the graceful and the robust.

As their name implies, graceful austrolopithecines are characterized by being more delicate and having smaller structures. The forehead is narrow and the sagittal crest is absent. The level of prognathism is varied.

In contrast, the robust variants are characterized by a broad cranial shape and have practically no forehead. The sagittal crest is present and the jaws are powerful. Little prognathism.

Australopithecus anamensis

Fossil bones at the Royal Belgian Institute of Natural Sciences, Brussels. By Ghedoghedo, from Wikimedia Commons

Australopithecus afarensis

A. afarensis

It dates from 3.75 to 2.9 million years ago and inhabited the Ethiopia, Kenya and Tanzania regions of East Africa. The skeleton - and the shape of the pelvis - allowed us to conclude that Lucy was able to walk upright.

When the fossil was discovered it was listed as one of the best preserved to date. The specific epithet of the species comes from the Afar tribe, who inhabited the locality where the fossils were found.

The cranial box of this species represents a third of the capacity of an average human, between 380 and 450 cubic centimeters. It has small sagittal chalk.

Regarding the size of the individuals, the males were much larger and more robust than the females.

Australopithecus africanus

Australopithecus Africanus skull lifeder. Tiia Monto, from Wikimedia Commons

This fossil dates between 3.3 and 3.5 million years. It was found in southern Africa and, like the previous fossil, it could move on foot in a bipedal way. In fact, the skeleton is quite similar to Lucy's.

The fossil teeth are very similar to those of modern humans, highlighting the small size of the canines and incisors. The separation between these two teeth disappears or decreases significantly.

Australopithecus garhi

National Museum of Ethiopia: Australopithecus garhi skull reconstructed from items found in 1997 (Awash region, Afar). 2.5 million years. By Ji-Elle, from Wikimedia Commons

This hominid fossil was found in regions of Ethiopia, and dates back to about 2.5 million years ago. The discovery was so unexpected that they used the specific epithet "garhi", which means surprise.

The size of the cranial box is comparable to that of other australopithecine specimens.

The species is characterized by the making of tools using rocks, being older than the tools found in Homo habilis.

Paranthropus (Australopithecus) aethiopicus

The Paranthropus aethiopicus fossil hails from Kenya, Ethiopia, and dates from 2.8 to 2.3 million years. It is one of the species considered "robust" of Australopithecus. For this reason, some authors argue about gender identity.

It is characterized by having strong jaws to be able to chew the tough vegetables that were part of its diet. They were strictly vegetarian species. Its jaws and associated muscles were so powerful that they resemble those of a modern gorilla.

Paranthropus (Australopithecus) boisei

The gender

Physical and biological characteristics

The genus Homo has a series of diagnostic characteristics (features that allow its identification and differentiate it from other groups).

The most striking feature is the increase in size of the brain - when compared to the ancient australopithecines. The volume of the box varies from 600 cubic centimeters to 2000 cubic centimeters in some H. sapiens.

With respect to the oldest groups, there is evidence of a reduction in the size of skull structures, such as the jaws and a general reduction in the face. Gender survival is largely based on adaptations at the cultural level. These include the tools they use, the discovery of fire, and the tendency to hunt.

The pronounced sexual dimorphism of the mentioned fossil species diminishes in Homo, where the differences between males and females are not so evident.

The genre is characterized by extreme flexibility in its ethology, which manages to adapt to a great variety of circumstances and problems. The most outstanding fossils of Homo are:

Homo habilis

Facial reconstruction of a Homo habilis.

In a fossil that inhabited Africa, specifically Tanzania, Kenya and Ethiopia, about 2.1 and 1.5 million years ago. It is considered "skilled" as there is evidence of possible tools and utensils made by such individuals. Its membership in the genus Homo is controversial by certain researchers.

Homo ergaster

Source: By Bjoertvedt, from Wikimedia Commons

It is a fossil native to South Africa, Ethiopia, which lived 1.9 to 1.4 million years ago. Of this species a skeleton in excellent condition of a child of approximately 11 years is known. Compared to previous Homo fossils, the skull has lost robustness. In terms of size, they were similar to humans today.

Homo georgicus

Fossil native to Georgia, Caucasus, which lived 2.0 to 1.7 million years ago. It is estimated that their height rarely exceeded 1.50 cm.

Homo erectus

Source: By Cicero Moraes, from Wikimedia Commons

There are a large number of characteristics that anthropologists use to characterize H. erectus, however the most conspicuous are:

Homo naledi

By Cicero Moraes (Arc-Team) et alii, via Wikimedia Commons

It is a hominid fossil that lived about 2 million years ago in South Africa. It is a relatively new species, it was described in 2014 using 15 individuals found in a chamber.

Homo heidelbergensis

By Tim Evanson, via Wikimedia Commons

This fossil species lived about 600,000 years ago, in European regions. They were characterized by being tall: males averaged 1.75 meters, while females reached almost 1.60 cm.

Homo neanderthalensis

Source:, via Wikimedia Commons

Neanderthal man is a species of hominin that lived approximately between 230,000 and 28,000 years ago, in the regions of Europe and Asia.

Neanderthals bear a slight resemblance to modern Europeans. However they were much sturdier and the limbs were shorter. It seems that the sense organs were highly developed. The evidence suggests that they may have had an articulate language.

Regarding their diet and food, they consumed a wide variety of fish, shellfish and vegetables - since they had the ability to hunt them.

In reconstructions they are usually represented with white skin and red hair. These traits are adaptive, since they inhabited regions of Europe and Asia, they needed to capture enough ultraviolet light - essential for the synthesis of vitamin D.

In contrast to individuals living in Africa. Melanin levels help protect against the high radiation to which they are exposed

Thanks to genetic analyzes, there is no doubt that there were repeated hybridization events between H. sapiens and Homo neanderthalensis.

Several hypotheses have been proposed to explain the extinction of this group: one of them is climate change, and another is related to competitive interactions with Homo sapiens.

Homo sapiens

Source:, via Wikimedia Commons

H. sapiens constitutes the current human species. It is characterized by colonizing virtually all terrestrial environments on the planet. Its cultural development, and its intellectual capacities and language development, differentiate it from the rest of the species.

Morphologically there are certain apomorphies (characteristics of a group) of the Homo sapiens species, the most outstanding are:

A globular shaped cranium with a vertical forehead, pronounced jaw, general loss of robustness in the body, crowns of the teeth decrease in size, with a reduced number of cusps and roots.

In terms of body structure, the limbs are elongated relative to the individual's trunk and body mass decreases relative to height. In the hands, the thumbs are elongated and the rest of the fingers are shorter.

Finally, there is a reduction of the hair that covered the body. The spine is S-shaped and the skull is balanced in the spine.

Where did humans come from?

The most widely accepted hypothesis is African origin. When we evaluate the genetic diversity of humans, we find that approximately 85% of all diversity can be found on the African continent, and even in a single village on it.

This model agrees with a case of the well-known “founder effect”, where only a small number of inhabitants leave their population of origin, carrying only a small variation of the population - in other words, it is not a representative sample.

References

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