Evolution is change in the heritable characteristics of biological populations over successive generations. A burning question arises overtime that how human being is evolved ?
Homo sapiens or Human being belongs to the class Mammalia which is evolved from an extinct species of fish called Tiktaaliks. It is very difficult for a common man to believe on this theory but there are authentic evidences which prove the evolution of man from fish.
The circumstances which caused the Tiktaaliks to migrate from ocean to the land include predation and lack of food. This theory was proved by the confirmation of Hox Genes both present in fish and mammals. Hox genes, a subset of homeotic genes, are a group of related genes that control the body plan of an embryo along the head-tail axis.
After the embryonic segments have formed, the Hox proteins determine the type of appendages or the different types of vertebrae that will form on a segment. The second evidence is hidden in Polyploidization or Whole Genome Duplication (WGD).
Polyploidization concept is supported by Ohno’s 2R hypothesis which states that “the evolution of vertebrates from a much simpler invertebrate ancestor was made possible by two separate rounds of whole-genome duplication during an early evolutionary period.”
Polyploidization is an extreme case of genome duplication and occurs only rarely during evolution. Duplication of a gene can probably occur by several different mechanisms but is most often thought to be produced by a process of unequal crossing over.
Genetic Evidence- Evolution of Globin Genes
Hemoglobin is a tetramer composed of four globin polypeptides. Examination of globin genes, whether from a mammal or a ﬁsh, reveals a characteristic organization. Each of these genes is constructed of three exons and two introns.
Examination of genes encoding certain globin-like polypeptides, such as the plant protein leghemoglobin and the muscle protein myoglobin, reveals the presence of four exons and three introns.
This is proposed to represent the ancestral form of the globin gene. It is thought that the modern globin polypeptide arose from the ancestral form as the result of the fusion of two of the globin exons about 800 million years ago. A number of primitive ﬁsh are known that have only one globin gene, suggesting that these ﬁsh diverged from other vertebrates prior to the ﬁrst duplication of the globin gene.
Following this duplication approximately 500 million years ago, the two copies diverged by mutation to form two distinct globin types, a type and a type, located on a single chromosome.
This is the present arrangement in the amphibian Xenopus and in zebrafish. In subsequent steps, and forms are thought to have become separated from one another by a process of rearrangement that moved them to separate chromosomes.
Each gene then underwent subsequent duplications and divergence, generating the arrangement of globin genes that exists today in humans. These evidences prove the arrival of present mammals from fish.
As humans are closely related to primates, so our main focus is on genus Homo.
Evolutionary journey from Homo ergaster to Homo sapiens
Huge debates rage about human origins, but the broad consensus among scientists is that all the different species of human that have ever existed were descended from ape-like creatures that walked upright in Africa more than six million years ago.
These creatures had many descendants, most of which became extinct, but the first creature we would recognize as human first appeared in Africa two million years ago, known as Homo ergaster, they made tools and were proficient hunters. Their bones suggest that they were powerful runners. H. ergasterseems to have evolved during a long period of terrible drought which dried out tropical rainforests and created vast deserts.
This human species was endowed to cope with heat. They would have been smooth and largely hairless, allowing them to sweat more efficiently. H. ergaster could also travel and hunt in the middle of the day, when most animals rest.
A hungry meat eater, ergaster became the first human to leave Africa and colonize Asia. Here he found the lush environment and evolved himself with a new name Homo erectus. According to Archaeological records they dispersed through an area ranging from Turkey to China with little population.
As Professor Chris Stringer, an anthropologist at the Natural History Museum said “These were small groups of hunters and gatherers”. Homo erectus had large face and teeth, strong brow ridge, long and low skull, powerful legs and brain 2/3 of human size.
Paleontological and Archeological studies show that both Homo Neanderthals and Homo sapiens share a common ancestor i.e. Homo erectus about half million years ago.
Neanderthal anatomy differed from modern humans in that they had a more robust build and distinctive morphological features, especially on the cranium, which gradually accumulated more derived aspects particularly in certain isolated geographic regions.
These include shorter limb proportions, a wider, barrel-shaped rib cage, a reduced chin, sloping forehead, and a large nose, being at the modern human higher end in both width and length, and started somewhat higher on the face than in modern humans.
The Neanderthal skull is typically more elongated and less globular than that of anatomically modern humans. Evidence suggests Neanderthals were much stronger than modern humans, with particularly strong arms and hands.
A 2013 study of Neanderthal skulls suggests that their eyesight may have been better than that of modern humans, owing to larger eye sockets and larger areas of the brain devoted to vision.
Neanderthals made stone tools, used fire and were hunters. The size and distribution of Neanderthal sites, along with genetic evidence, suggests Neanderthals lived in much smaller and a few distributed groups.
The modern man is much like H. Neanderthals and not like Chimpanzees but we don’t yet know for sure, but it seems likely that, as part of their adaptation to cold, Neanderthals were furry.
Chimpanzees have ridges on their finger bones that stem from the way that they clutch their mothers’ fur as infants. Modern humans don’t have these ridges, but Neanderthals do. This observation shows the evolutionary relationship of primates.
Evolution of Homo sapiens
Modern humans, or Homo sapiens, and Neanderthals shared a common ancestor roughly half a million years ago. They then split and evolved in parallel: humans in Africa, and Neanderthals on the Eurasian continent (Eurasia is a combined continental landmass of Europe and Asia).
When humans finally ventured to Eurasia, they had sex with Neanderthals, swapping DNA around. Today, people who aren’t of African descent owe roughly 2 percent of their DNA to their Neanderthal ancestors, while Africans do not have Neanderthals DNA. Homo sapiens are the single remaining species on Earth now in present days.
The genetic analysis shows that modern man must be mated with H. Neanderthals and Denisovans which evolved into Homo floresiensis (Hobbits). These small humans were ultimately evolved into Early Modern Man (EMM). These EMM then modified into modern Homo sapiens in course of thousands of years.
Why Homo sapiens survived to populate the globe?
There is an interesting questions arises from above discussion that our ancestors were more powerful than us. Then what were the factors which caused them to extinct and left us modified?
There are a lot of theories to solve this mystery including that alliances between modern humans and dogs helped humans hunt food better, essentially starving Neanderthals out of Europe. Or, humans might have reproduced faster than Neanderthals, multiplying and edging them out.
To piece the story together, scientists are searching for more Neanderthal genomes locked in ancient bones, and for more Neanderthal DNA hiding in present-day genomes.
There are two studies which will help us to reveal the reasons and adaptations of human beings. The first study published in SCIENCE and the second study was published in AMERICAN Journal of Human Genetics.
The first study, published in Science, describes a bone fragment called Vindija 33.19, which was found in a Croatian cave of the same name in the 1980s. Now, researchers have finally been able to sequence the DNA locked inside, discovering it belonged to a female Neanderthal who lived 52,000 years ago.
Researchers found that the Vindija Neanderthal was very similar genetically to another Neanderthal who died about 122,000 years ago in the Altai Mountains of Siberia. The fact that two Neanderthals separated by more than 3,700 miles and 70,000 years were so similar suggests that Neanderthal communities were tiny, with very little genetic diversity.
As according to Kay Pruefer “It’s quite amazing when you think about it. They are really so closely related that you cannot find any two people on this planet that are this close”.
That could support the theory that Neanderthals’ low genetic diversity may have contributed to their extinction. Genetic diversity forms the basis for natural selection. If everyone in a population had the exact same versions of the same genes, then one plague or one hard winter could wipe everyone out.
And then there’d be no survivors to pass on the genes that would give their offspring a chance to survive the next plague or harsh winter. In the second study the researchers found that Neanderthal DNA was especially common in stretches of genes that contribute to hair color and skin tone.
Some Neanderthal variations were associated with blonder, paler complexions, and others were with darker pigmentation. That could mean that Neanderthals themselves had different hair and skin colors.
These results don’t mean that Neanderthal genetic mutations cause these traits in modern-day humans, or that the same genetic variations gave rise to the same traits in Neanderthals.
A third study published in Science reveals that Early Modern Humans lived in large, complex social networks that helped them swap genes and ideas. Early modern humans weren’t just having sex with Neanderthals, of course. They were also banging each other.
From the above paragraphs, we can conclude that Homo sapiens had mated with Neanderthals and Denisovans and hence their genetic makeup became more diverse than their ancestors did.
As they had the abilities to face the warm conditions while adopted abilities against harsh cold weathers after mating with Homo Neanderthals which were genetically modified for winters. Their grasp to food sources and complex social networks triggered them to modify their genome via natural selection and genetic drift.