Human Evolution

Humans are known as Homo sapiens (modern man). The full classification is:

Kingdom:Animalia

Phylum:Chordata

Subphylum:Vertebrata

Class:Mammalia

Subclass:Eutheria

Order:Primates

Suborder:Anthropoids

Family:Hominidae

Genus:Homo

Species:Sapiens

The fossil record, allows us to look at common morphology and deduce common ancestry. The relatedness of organisms is investigated by comparative biochemical studies, particularly of mitochondrial DNA, which in each generation is passed from mother to offspring unchanged. This type of DNA undergoes a steady rate of mutation – it changes as a function of time alone. The degree of difference between mitochondrial DNA samples discloses how recently groups of organisms shared a common ancestor.

Dating Rocks and Fossils

Fossils can tell us a lot about the past.

Fossil – any form of preserved remains from a living organism.

Some examples are:

  • Mammoths frozen in Siberia
  • Mummies in acidic swamps in Scandinavia
  • Insects in amber
  • Bones in rock

Fossils are only formed in some circumstances. Most individuals do not leave a fossil after death.

A fossil has to be formed when an organism dies and gets buried in sedimentary silt. It will decay slowly and leave a space in the silt. The gap becomes solid and is filled the exactly the same as the organism left behind. The silt may solidify, becoming sedimentary rock and in it is the fossil.

To see how old fossils are and their forms, carbon dating is used, usually Carbon 14 and potassium 40, which are isotopes.

Isotopes are atoms of the element that have different numbers of neutrons. Therefore, they are unstable and will spontaneously change into one or more atoms to other elements, often emitting some radiation. The time taken for this change is determined by the kind of isotope. After a period of time, at a fixed interval, the radioactive decay will be half of what it was before. This is called half-life. For C14, the half-life is 5730 years.

Using C14

Most carbon is C12, but due to cosmic radiation C14 is formed at a low, steady rate. While alive, organisms absorb carbon in the ratio of C12 / C14 present in the environment around them. After death, accumulation of radioactive (and other) atoms stops. Meanwhile, C14 steadily breaks down:

half-life of 5.6 X 103years

C14N14

So the ratio of C14: C12 in a fossil decreases with age; the less C14, the older the fossil.

This technique gives good dates for fossils of the last

60 000 years.

Using the ratio of K40:Ar40

Rocks do not eat or photosynthesize. Some contain no carbon at all. Instead we use potassium – 40. The pyroclastic rocks flowing out of volcanoes may contain radioactive isotopes such as potassium-40, which decays to argon-40, as shown:

half-life of 1.3 X 109yrs

K40 Ar40 (gas)

In hot lava, argon gas boils away into the atmosphere. Once lava has solidified by cooling, which occurs quickly after volcanic eruptions, the argon gas that is then formed by radioactive decay is trapped in the rock.

By measuring the ratio of K40: Ar40 in lava deposits, the exact ages of the lava and the approximate age of the sedimentary rocks (and their fossils) below and above lava layers are estimated.

This technique spans the whole of geological time back to the Cambrian period (580 million years ago), but it is too slow to give reliable results over the most recent half million years.

Determining the age of a fossil or rock.

You look at the percentage of Carbon – 14 or potassium – 40 left in the fossil or rock. If there is 50% left, that means there has been 5730 years past for C14. For 25%, that would be double the half-life or 11 460 years. Another way, is to look at a decay curve and see where the amount of remaining C14 falls on the curve to estimate the time.


Humans as Primates

Humans belong to the mammalian order Primates. This order contains three distinctive groups of animals, namely the apes (which includes the genus Homo), the monkeys, and the prosimians (a name meaning ‘before the monkeys’). These are mostly tree-dwelling species with grasping hands and feet. The range of animals that constitute the Primates and how they are related are summarized below.

Apart from humans, who have achieved worldwide distribution, most primates live in tropical and sub-tropical regions. An interesting feature of primates is their relatively unspecialized body structure, combined with some highly sophisticated behavior patterns.


Why are humans defined as primates?

To the biologist, humans are primate mammals. By this we mean that humans show many of the characteristics of other mammals, the general characteristics common to other primates, and many of the features shown by the great apes to which we are most closely related.

Major features, which describe humans, as primates are adaptations to tree life. They are the opposable thumb, acute vision and a large brain.

Having an opposable thumb means you can manipulate objects and be able to grasp. This is very important for tree dwellers. Also, living in trees means you can see further. As a result, the eyes are places more forward. This gives a smaller field of vision, but more acute and the overlap of vision allows for good depth perception and judgment of distance. Along with this is color vision. Lastly, primates have large brains that improve the ability to manipulate items and as primates look after their young for a long time, the young can learn many skills from the adults.





Origins of Humans – Trends in hominid and human fossils

The earliest fossils which we confidently identify as anthropoids (apes) have been found at many sites in Africa. They date from about 35 million years ago (mya). Humans clearly demonstrate one form of anthropoid body organization, so we can say the human story has taken about 35 million years to unfold.

There are some similarities and some major differences between all the hominid skulls that have been unearthed. The details to pay close attention to are summarized below. One key area is the foramen magnum. It is the hole where the skull is attached to the spinal column. In modern humans the hole is in the center of the base of the skull, giving rise to the theory of walking upright. In apes, the hole is further back to accommodate the spinal column in an animal that walks on four legs.

There is some discussion as to what a hominid is. It can only refer to those bipedal primates, which are direct ancestors of modern humans.


The chronological order of some of the species of hominid for which have been found is below.

Fossils of Hominidae have lead to several speculations about evolution.

  1. Ardipithicus ramidus
  2. Lived approximately 5.8 – 4.4 mya in Ethiopia. This species is believed to be very close to the split between the line of organisms, which became more human-like and the line, which became more chimpanzee-like. Most of the fossils are teeth and therefore, it is difficult to be sure. From what has been found, the Ardipithicus ramidus was very similar to a chimpanzee with a few hominid features. The molars show more ape-like characteristics, as the length is greater than the breadth. The canines are more hominid, as they are shorter and not as sharp as ape canines.
  1. Australopithecines (southern ape) lived about 4 mya. They had 500 cm3 brains and walked upright.
  2. The first species was A. afarensis from the Afar desert (4-2.8 mya) found in Ethiopia and Tanzania. (Lucy skeleton) It had a tall lower jaw, fairly large molar teeth and a projecting face. The cranial capacity was 380 – 430 cm3.
  3. Later came africanus (3-2 mya) found in South Africa. It is thought to be the same species as afarensis as features are similar and walked upright. It had a tall, thick lower jaw, large molars and a projecting face. The cranial capacity was 435 – 530 cm3.
  4. Later was A. robustus (2-1.4 mya) in South Africa. They were larger and heavily built.
  1. Then came the Homo genus. They were from around 2 mya and had larger brains (600 cm3) and walked upright.
  2. First was H. habilis (handy man). It is thought he arose from A. afarensis 2 mya in East Africa and used simple tools. It had a flatter face, larger molars but the cranial capacity was still only about 600 cm3.
  3. Homo erectus was from Africa. It is thought it migrated to other parts of the world and had a larger brain than H. habilis. H. erectus spread to Asia and Europe. However, it is believed that H. sapiens evolved at one place in Africa and from their spread out over the world. It has a smaller jaw, a receding forehead, large brow ridges and smaller molars. Its cranial capacity was 1000 cm3.
  4. H. neanderthalensis, which lived in Eurasia from 200 000 to 30 000 years ago. The species survived several ice ages. It has a smaller jaw, a lower forehead, smaller brow ridges and smaller molars than the previous species. They had larger brains than modern humans, with a cranial capacity of up to 1600 cm3.
  5. Next was H. sapiens, which came to Europe. The first subspecies was Cro-Magnon man, who looked a lot like modern humans and though to have used the first language. H. sapiens lived around 140 000 to 70 000 years ago in Africa and Asia as well. They had a high forehead, no brow ridges, a flat face, small molars and a very small jaw. This species developed cave paintings, tools and weapons. The cranial capacity was similar to today’s humans of 1300 cm3.


Based upon where the skulls were found and dating, we can see that many species may have coexisted. As the Homo sapiens developed, so were the homo neaderthalenis.

The incompleteness of the Fossil Record

Anthropologists disagree about the origin of modern humans from time to time. They use evidence from fossil remains, from artifacts like stone tools that can be associated with particular hominids, and the record in animal bones that surrounded their habitations and which indicate diet. Fresh evidence of these types is frequently discovered, and existing data are sometimes reinterpreted.

Re-interpretation occurs in the light of new biochemical evidence or the development of new analytical techniques. For example, until quite recently, another theory about the origin of modern humans vied with the current ‘out of Africa’ theory.

The alternative was a multiregional model, in which H. sapiens emerged wherever populations of H. erectus had become established, in Africa, Europe and Asia. This made H. neanderthalensis only one example of an archaic hominid form, intermediate between H. erectus and modern humans. According to this model, there was ongoing genetic exchange between populations of various archaic forms until H. sapiens emerged and replaced all others. Currently, the body of evidence is increasingly against this theory.

Controversy will continue because of the inevitable incompleteness of the fossil record. Fossilization is an extremely rare, chance event. This is because predators, scavengers and bacterial action normally break down dead plant and animal structures long before they can be fossilized. Of the relatively few fossils formed, most remain buried, or if they do become exposed, are often overlooked or may be accidentally destroyed before discovery.

Nevertheless, numerous fossils have been found, and as more hominid fossils are discovered, so our knowledge may change and our understanding of our past be advanced. This is yet one more branch of science where the frontier of knowledge is entirely open. You can follow the debate from now on.

Brain Size

So why did the brain develop? Some think because the environment was so diverse, a larger brain was needed to deal with the challenges, and therefore, larger brains were selected as an advantage.

Habilines were the first hominids to be associated with tools – they used large pebbles, chipped in at least two directions, as sharpened implements to crush, break and cut. Their additional brain capacity had resulted in advanced manual dexterity. It was applied to the making and using of simple tools (selected strong stones) to chip pebbles, for a purpose. Using tools to make tools (i.e. the development of a tool industry) is what distinguishes hominid toolmakers from all other tool-users in the living world.

Skull endocasts (casts of the inside of the brain case of the skull) show that the areas of the brain associated with speech and language are significantly developed, so we can assume that cultural evolution was also under way. This was also the first hominid to use fire consistently, which will have aided the colonization of areas so far north of equatorial Africa, and also with its habit of eating meat.

By modern human standards, H. erectus had a marked brow-ridge and protruding jaws, but the pronounced sexual dimorphism of earlier hominids was reduced – adult males were now only about 20–30% larger than females.

The brain size of Neanderthals was larger than that of modern humans. This may reflect the requirements of controlling the large musculature, because they were heavier and more muscular than H. sapiens. The latter are more slightly built, but taller and longer-limbed.

Humans were hunter–gatherers but compared with many of the competing wild animals, not especially strong or fast. Scavenging would have been a major source of nutrients, at least initially. Only with the development of agriculture and other advances in technology (e.g. brewing, cheese making) did humans move into circumstances in which population sizes grew significantly, and they could start to dominate their environment and become secure.

The issue of the actual diet at each stage may have been a critical factor. This is because brains are metabolically expensive. Our brains make up 2% of our body mass but respire about 20% of our energy budget. The human brain is about three times the size of that of an equivalently sized ape. We may conclude that the expansion in the brains that we have noted in the succeeding species of Homo will have demanded enhanced energy supplies. This means that human evolution must have been increasingly dependent on a reliable supply of protein and fat. However, it is not dependent on advanced hunting skills, at least not from the outset. Hominids will have discovered that the long bones of herbivorous mammals (discarded by the large carnivorous mammal hunters around them) were a rich source of bone marrow. Bone marrow is rich in protein and fat and could be accessed by nothing more sophisticated than a heavy blow from a rock onto the bone shaft, held against a hard place.

Bipedalism

Parts of skulls of this genus have been uncovered in various locations (including at Taung, in 1924), prior to the discovery of a new hominid fossil, first known as Lucy, at Hadar in Ethiopia in 1974.

Lucy is identified as Australopithecus afarensis. She was ape-like in that she had the same limited brain capacity as ape species of the period, but hominid-like in that she was a powerful, upright walker (the pelvis was of characteristically human form) and had no long muzzle.

We now recognize that upright walking (known as bipedalism) was an early stage in the evolution of the hominids. The Lucy fossil was laid down 3 mya.

We are confident about bipedalism at this time, because of the discovery of the footsteps at Laetoli, imprinted in volcanic ash, 3.6 mya. The soft ash was presumably moistened by rain (no additional prints added), immediately baked into hard rock, and then buried by soil blown in. The footsteps were discovered in 1976. Two adults had walked in line, in a northerly direction, with a youngster who later ran off to one side. Being volcanic ash, this trace fossil can be dated precisely by the potassium:argon ratio method.

One advantage of bipedalism (perhaps the chief advantage, initially) is as a mechanism to prevent the head region of the body overheating at the high midday temperatures of equatorial latitudes. Being upright does not expose as much surface area to the sun. Australopithecines lived in mosaic environments: part tropical rainforest, part woodland and tree-savannah, part scrub. Wherever they lived, no doubt they preferred to shelter at times of greatest temperature. But they may have often needed to travel to new venues, visit water holes, or scavenge and collect food at times when faster and stronger predatory animals were most likely to be resting. If so, being bipeds gave them an advantage.

Another critical advantage of bipedalism is that hands are freed for obtaining and carrying food. Apes breed slowly, producing few offspring at a time. A male ape that had mastered bipedalism could improve his mate’s reproductive capacity by feeding her, thus freeing her to concentrate on the production and rearing of young. The genes of apes with a tendency for bipedalism will have had a better chance of replication in future generations. This would have been particularly effective in male–female pairs, rather than in troops of primates where males invested time and energy maintaining dominance over the females. On this account, hominids would have tended to be monogamous apes with lessened sexual dimorphism (males the same size as females).

Genetic vs. Cultural Evolution

Genetic evolution refers to the changes in allele frequencies that result in changes in individuals and therefore in populations, brought about by natural selection.

In outline, these are due to:

  • Genetic variations, which arise via mutations, random assortment of paternal and maternal chromosomes in meiosis, recombination of segments of maternal and paternal homologous chromosomes during crossing over that occurs in meiosis in gamete formation, and the random fusion of male and female gametes in sexual reproduction.
  • When genetic variation has arisen in organisms, it is expressed in their phenotypes. Some phenotypes are better able to survive and reproduce in a particular environment, and natural selection operates to determine the survivors and the genes that are perpetuated in a population. In time, this process may lead to new varieties and new species.

By cultural evolution we refer to the development of the customs, civilization and achievements of people. The development and transmission of human culture has a biological basis.