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Human Evolution

An overview
by

Jessie Varquez

on 13 September 2016

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Transcript of Human Evolution

WHO WERE THE FIRST HUMANS?
An overview of human evolution
Jessie G. Varquez, Jr.
To recognize that we are the only surviving human species today - i.e.,
Homo sapiens
To acknowledge that we evolved from early hominin species through natural selection
To understand the two competing theories explaining the origins of modern humans
How many humans are there in the world today?
If we compare earth’s history to a

24-hour day

(with 1 second equaling 50,000 years), Earth originates at midnight;
the

earliest fossils

were deposited at

5:45 a.m
.;
the

first vertebrates
appeared at

9:02 p.m
.;
the

earliest mammals
, at
10:45 p.m
.;
the

earliest primates
, at
11:43 p.m
.;
the

earliest hominins
, at

11:57 p.m.
; and
Homo sapiens

arrives

36 seconds before midnight.
What makes us humans?
in relation with other primates
bipedalism
savanna hypothesis
= see over long grasses preys and predators;

thermal regulation hypothesis
reduced body exposure to solar radiation;
provisioning hypothesis
= the male needs to provision the female to lessen the period of infant dependency and thus making the female available and sexually viable;
migration hypothesis
= increased the amount of food found over greater distances;
posture hypothesis
= the erect posture of hominids allow for the ability to evade predators and display aggression and threat which is more effective than being quadruped;
reproductive hypothesis
= erect posture is conducive for reproductive efficiency. Hominid’s copulation position (ventral-ventral) allows the female reproductive tract to be parallel with gravity, thus greater sperm retention around the cervix;
birthing hypothesis
= made the birthing process a social experience. Assistance from fellow hominid was necessitated because females are vulnerable in the birthing process
why did we become bipeds?
ano sa palagay mo?
free hands
to carry items back to a home base and carry a weapon, thus killing games;
It is important to note however that bipedalism is an
evolutionary trade-off
- meaning it also entails negative adaptation - e.g., back pains and injury (arthritis), burdened circulatory system (heart needs to pump stronger) and incapacity to move if one leg is injured.
dentition
The churning, rotary motion associated with such chewing also favored reduction of the canines and first premolars.

Human teeth reflect completely omnivorous diet and are not very specialized
'nonhoning chewing; no slicing mainly grinding'
Larger skulls, enlarged brain, longer childhood dependency, and generally
social behavior
Much greater proportion of human behavior is
learned
and culturally patterned
Another distinctive human trait:
sexuality
of human females who may engage in intercourse at any time throughout the year
Humans are also unusual among the primates in having
female-male bonding
Only humans have
spoken, symbolic language
Natural selection

operates on these principles:
variation
,
heritability
,
and

differential reproductive success
.
Without variation, natural selection has nothing on which to operate; without variation, one kind of characteristic could not be favored over another.
Offspring inherit traits from their parents, at least to some degree and in some way.
Since better adapted individuals generally produce more offspring over the generations than the poorer adapted, the frequency of adaptive traits gradually increases in subsequent generations.
In October 1891, after moving his search to the neighboring island of Java, Dubois’ field crew unearthed a skullcap along the Solo River near the town of Trinil—a fossil that was to become internationally famous. The following year, a human femur was recovered about 15 yards upstream in what Dubois claimed was the same level as the skullcap, and he assumed that the skullcap (with a cranial capacity of slightly over 900 cm3) and the femur belonged to the same individual.
Six sites in eastern Java have yielded all the H. erectus fossil remains found to date on that island. The dating of these fossils has been hampered by the complex nature of Javanese geology, but it’s been generally accepted that most of the fossils belong to the Early to Middle Pleistocene and are between 1.6 and 1 million years old. But as we noted earlier, more precise chronometric dating estimates have suggested that the earliest site may be close to 1.6 million years old, and very late H. erectus survivors (from Ngandong) may be as young as 27,000 years old.
The story of the first discoveries of Chinese H. erectus is another saga filled with excitement, hard work, luck, and misfortune. Europeans had known for a long time that “dragon bones,” used by the Chinese as medicine and aphrodisiacs, were actually ancient mammal bones. .
Scientists eventually located one of the sources of these bones near Beijing at a site called Zhoukoudian. Serious excavations were begun there in the 1920s, and in 1929, a fossil skull was discovered.
The skull turned out to be a juvenile’s, and although it was thick, low, and relatively small, there was no doubt that it belonged to an early hominid. The response to this discovery, quite unlike that which greeted Dubois almost 40 years earlier, was enthusiastically favorable
The fossil remains of H. erectus discovered in the 1920s and 1930s, as well as some more recent excavations at Zhoukoudian, are by far the largest collection of H. erectus material found anywhere. This excellent sample includes 14 skullcaps, other cranial pieces, and more than 100 isolated teeth, but only a scattering of postcranial elements.
Both theories agree that Africa is the home of earlier forms of hominids; the contention arises on the question of
where exactly
did the modern humans (i.e.,
Homo sapiens
) emerge as a
distinct species
.
The Multiregional Model is closely associated with paleoanthropologist Milford Wolpoff of the University of Michigan. This theory argues
that modern humans are descendants of premodern Middle Pleistocene hominids (i.e., Homo erectus) in various local populations of the Old World
, not just in Africa but most especially in Asia and Europe.
Thus, Multiregional model denies that Homo sapiens populations derived exclusively in Africa alone. Furthermore, there were evolutionary processes of gene flow and natural selection.
Local populations of hominids would not have independently evolved from one another
, and this mixing of population would prevent speciation between regional lineages of hominids.
The strongest evidence of this Multiregional continuity model are
archaeological finds of Homo erectus fossils
in Asia and Europe. In the documentary Incredible Human Journey, the Chinese believe that they are the direct descendants of Homo erectus, advocated and preached by their scientist – Professor Wu Xinzhi, a staunch Multiregional defender.
Out-of-Africa model, on the other hand, was developed by British paleoanthropologists Christopher Stringer and Peter Andrews (1988). Simply stated, this theory proposes that “
modern humans evolved relatively recently in Africa, migrated into Eurasia and replaced all populations which had descended from Homo erectus
”.
An important component of this assertion is that the African
Homo sapiens that replaced other regional populations of hominids elsewhere were products of speciation
– they are distinct biological species, which means they could not have interbred with local non-African populations in other parts of the Old World.
The
strongest evidences of this theory are the genetic studies
made possible by advances in molecular biology. In 1987, geneticists Rebecca Cann and her colleagues published a landmark study that cemented the claim of African origin.
In popular discourse this is known as
Eve hypothesis or mitochondrial Eve
. In this study, they examined mtDNAs of 147 people from five geographic locations and claimed “All these mitochondrial DNAs stem from one woman who is postulated to have lived about 200,000 years ago, probably in Africa”. Because the genetic information of mtDNAs are passed on only from the mother line, they were able to trace most mutations or genetic variations from the African samples which they used as the basis of their conclusion.
For the moment, the majority of anatomical, archaeological and genetic evidence gives credence to the view that fully modern humans are a relatively recent evolutionary phenomenon.

The
current best explanation
for the beginning of modern humans is the
Out of Africa Model
that postulates a single, African origin for Homo sapiens
.

The major neurological and cultural innovations that characterized the appearance of fully modern humans has proven to be remarkably successful, culminating in our dominance of the planet at the expense of all earlier hominid populations.
Conclusion
(Johanson 2001)
One of the criticisms about Multiregional theory is that it fuels the dangerous idea of
racism
,
since modern day human populations have evolved in various regional populations which accounts for their phenotypic diversity.
Further notes

Thus,
Out-of-Africa enjoys wide popularity
among scholars and the public. However, both theories should be seriously considered in light of our understanding of modern human dispersion.

Aside from the advances of genetic research, the present day repertoire of
hominid fossil evidence
may favor certain arguments of one particular camp of the debate.

But if fresh finds will be soon available, the balance might be tilted towards the other camp, more so revising our
basic understanding of human evolution
, if not a total overhaul.
Tanong? Paglilinaw?
References
Most texts and photos were taken from:
Jurmain, Robert, et.al. 2006.
Essentials of Physical Anthropology
, Seventh Edition. Belmonth, CA: Wadsworth.
Larsen, Clark Spencer. 2010.
Essentials of Physical Anthropology: Discovering our Origins
. New York and London: W.W. Norton & Company.
Additional texts from:
Ember, Carol R. and Melvin Ember. 1999.
Anthropology
, Ninth Edition. Singapore: Prentice Hall.
Johanson, Donald. 2001. "Origins of Modern Humans: Multiregional or Out of Africa?" http://www.actionbioscience.org/evolution/johanson.html
Kottak, Conrad Phillip. 2010.
Mirror for Humanity, A Concise Introduction to General Anthropology
, 7th Ed. New York: McGraw-Hill.
I do not own any material used in this Prezi. This is strictly for academic use.
Assignment!
If a small group of modern humans left Africa to populate the rest of the world, which route did they take?
Full transcript