Download Human Evolution

Human Evolution
Assessment Statement
D.3.1
D.3.2
D.3.3
D.3.4
D.3.5
D.3.6
D.3.7
D.3.8
D.3.9
D.3.10
Outline the method for dating rocks and fossils using radioisotopes, with
reference to 14C and 40K
Define half-life
Deduce the approximate age of materials based on a simple decay curve
for a radioisotope
Describe the major anatomical features that define humans as primates
Outline the trends illustrated by the fossils of Ardipithecus ramidus,
Australopithecus, including A. afarensis, and A. africanus, and Homo,
including H. erectus, H. neanderthalensis and H. sapiens
State that, at various stages in hominid evolution, several species may
have coexisted
Discuss the incompleteness of the fossil record ant the resulting
uncertainties about human evolution
Discuss the correlation between the change in diet and increase in brain
size during hominid evolution
Distinguish between genetic and cultural evolution
Discuss the relative importance of genetic and cultural evolution in the
recent evolution of humans
Humans are known as Homo sapiens (modern man). The full classification is:
Kingdom:
Phylum:
Subphylum:
Class:
Subclass:
Order:
Suborder:
Family:
Genus:
Species:
Animalia
Chordata
Vertebrata
Mammalia
Eutheria
Primates
Anthropoids
Hominidae
Homo
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 103 years
C
14
N14
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:
K40
half-life of 1.3 X 109 yrs
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 (stereoscopic vision), mobile arms and shoulder
girdle and a skull modified for upright posture.
Having an opposable thumb means you can manipulate objects and be able to grasp.
Mobile arms, allows movement in three planes and transfer weight via the arms. This is
very important for tree dwellers and for movements above the head. Also, living in trees
means you can see further. As a result, the eyes are places more forward, on a flat
front. 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. The positioning of the magnum foramen (mentioned later in these notes),
allows for the spinal column to insert into the skull in a more upright position, lessening
the curvature of the spine and allowing primates to walk upright.
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
a. 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.
2. Australopithecines (southern ape) lived about 4 mya. They had 500 cm3 brains
and walked upright.
a. 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.
b. 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.
c. Later was A. robustus (2-1.4 mya) in South Africa. They were larger and
heavily built.
3. Then came the Homo genus. They were from around 2 mya and had larger
brains (600 cm3) and walked upright.
a. 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.
b. 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.
c. 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.
d. 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 longerlimbed.
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.
Key to this was the extension of the period of parental care, delayed onset of puberty,
and the resulting long period of childhood when the next generation of a population are
trained and schooled as they develop essential survival skills – all features of the
evolution of the genus Homo.
The development of language is the most important human characteristic central to the
evolution of culture. Endocasts give a slight impression of the areas of the brain that
developed and were enlarged (the chief neural machinery for speech in most modern
humans is found in the left hemisphere). Also critical is the position of the vocal folds in
the neck. On both counts it seems likely that only Neanderthals and H. sapiens achieved
the structures necessary for elaborate vocal communication. In particular, the high palate
and high larynx found in H. sapiens allowed a greater range of resonance for complex
word sounds.
Once established, verbal communication allowed advantageous developments (for
example, in the form of new ideas) to be passed on rapidly. The potential speed of
development of this form of cultural evolution contrasts markedly with change brought
about by slow inherited accumulation of advantages by genetic evolution. Today’s latest
cultural-sharing breakthroughs – the Internet and the human genome project – are cases
in point.
The developments in tool technology were also dependent on the development of a
large brain.
Compared to the achievements of the Habilines in this, from about 35 000 years ago,
modern humans made spectacular advances. Bone and antler were added to the list of raw
materials, and advances in the skills of fashioning stone flakes and blades into finely
worked scrapers, chisels, drills, arrowheads and barbs were spectacular. Tool-kits
comprised items for engraving and sculpture. Functional implements like spears became
decorated with life-like animal carvings.
The latter point relates to human use of the brain, powers of detailed observation, and
manual dexterity, all of which underpin cultural development. Homo sapiens as observers
and artists achieved incredible feats at the earliest phase of their development.
We have a remarkable record of the artistic skills of our first human ancestors in the cave
paintings from this period that have been discovered. The drawings, produced by human
communities from 25 000 to 10 000 years ago, show contemporary animals in scientific
detail. The pictures demonstrate perspective representation.
The relative importance of genetic and cultural evolution is quite obvious.
Genetic evolution has given rise to the diversity of living things, including human beings.
However, this is a process that has taken thousands of millions of years. The special
features that humans have developed, mostly unique to them, have been the basis of
cultural evolution. For example, with the development of agriculture and other
technologies, humans have changed their immediate environment with the creation of
settlements and then gone on to evolve communal living. Enlarged populations have been
both necessary to the new way of life, and sustained by it. Rules and laws have succeeded
basic customs, and individuals have acquired rights and responsibilities. Consequently,
the conditions for genetic evolution have been progressively sidelined as the processes of
cultural evolution have taken over.