How Long Have Humans Been on Earth?
One of the most commonly asked questions is how long have humans been on the Earth. The Biblical genealogies indicate that human beings have been around the planet for roughly 4,000 years, from the time Adam was born until Jesus of Nazareth. This question is complex and has been the subject of much debate. Here are a few points to consider when answering this question.
About 6 Million Years
Homo sapiens is thought to have migrated from Africa in waves approximately 6 million years ago and later reached southern Asia, Oceania, and Europe. This group of people interbred with the ancient, archaic human populations in Europe and Asia. However, the origin of fully modern humans is disputed.
The earliest known fossil remains of modern humans, the Omo I and Omo II skulls, were discovered in southwestern Ethiopia in 1967. They have been dated to about 195,000 years ago, making them the oldest anatomically modern humans ever found. These fossils are thought to be the ancestors of modern humans. These ancient human remains suggest that the first wave of humans emerged out of Africa about six million years ago and that this population dwindled to a mere 10,000 people.
Since that time, the evolution of the human species has been a continuous process. The human species has evolved over time to meet new environmental pressures, and its genetic diversity has grown steadily. This is a natural process, as the larger the population, the more genetic variation and mutations will occur.
During the evolution of humans, the cranial capacity of humans has increased steadily. Early human species, such as the Sahelanthropus, had a cranial capacity that is comparable to chimpanzees today. Later australopithecines, however, grew more sophisticated, with skulls of about four hundred and fifty cubic centimeters. By the time Homo erectus was developed, its brain was around twelve hundred and twenty-two centimeters in size.
Humans First Stepped on Earth
At approximately 635 million years since humans first stepped on Earth, sexual reproduction began to evolve. This allowed faster evolution due to greater genetic variation and selection. Early mammals were likely small shrew-like animals that fed on insects. During this time, they also began to develop their organs such as a three-chamber heart, glottis, and parathyroid glands.
Homo erectus began a migration through Eurasia, reaching Southeast Asia by 0.7 Ma. They were highly social and scavengers, but later became hunters. They also developed the ability to communicate. This may have been the catalyst for the expansion of the brain between two and eight million years ago. Homo sapiens also lost their brow ridges from their hominin ancestors and evolved a protruding nose. By 200 ka, however, brain growth stopped.
Evidence for ‘out of Africa’ Model
The Out of Africa model of human evolution has gained a lot of attention lately, thanks to a number of recent studies. The earliest evidence came from fossil evidence, but a new study shows that it is backed up by DNA analysis. Neandertals and modern humans have very different mitochondrial DNA sequences, which supports the ‘out of Africa’ hypothesis.
The ‘out of Africa’ model is based on the assumption that modern humans originated in Africa between 150 and 200 thousand years ago. They then spread throughout the Old World, replacing pre-existing archaic human species outside Africa, such as the Neandertals from Europe. The theory says that modern humans originated in Africa, and that they were replaced by populations from other regions.
However, this theory has been challenged by inconsistent evidence from Australia. For one, aboriginal Australians’ skulls show distinct traits compared to those of modern humans from other parts of the world. Further, the skeletal remains of early Australian settlers show distinct differences from those found elsewhere on the ‘coastal expressway’. As a result, some researchers claim that the early Australian settlers were probably part of an early replacement event from Africa.
Despite this new evidence, many experts still question whether the earliest human species actually left Africa. The fossil record of the early species of Homo sapiens shows that the species expanded its range into southern Eurasia. From there, the species moved eastward, establishing a presence in South East Asia.
Model is no Longer Popular
Although the ‘out of Africa’ model is no longer popular, it still has a strong place in multiregional human evolution. The continent was the main center for human evolution during the Pleistocene era and has contributed significantly to the gene pool of modern humans. The ‘out of Africa’ model also contends that African populations interbred with non-African populations long before modern humans emerged.
Recent discoveries have sparked debate on whether modern humans left Africa as early as 60,000 years ago. However, the oldest known fossils of Homo sapiens that were discovered outside Africa date between 90,000 and 120,000 years ago. This new discovery is further complicating the conventional model of human dispersal, which relies on two waves of Africa migrations. Further, genetic analyses have shown that modern humans are direct descendants of the second wave.
Population Growth During 8,000-year Period
The world population grew at an average rate of 0.05% a year during the past 8,000 years. This rate of population growth was largely a result of changes in food and water availability and periods of hostility and peace. In addition, the population of Earth increased because of the spread of agriculture.
Up to the mid-1700s, the global population grew at a relatively slow pace. While fertility was high, it was offset by high mortality among children. At that time, the world was in the first stages of a demographic transition. In addition, the world population was estimated to reach around four billion people by the year 2100, but this number is still high, despite the fact that deaths have decreased dramatically.
The slow growth was a combination of several factors, including increased death rates during the pandemic period, a declining fertility rate, and a slowing of immigration. While immigration has the largest contribution to the future of the population, it is also the reason for the slow growth. Public policies can overcome barriers to childbearing and stimulate immigration.
Another factor contributing to the growth of populations is the rate of environmental productivity. While the SPD does not capture high-frequency variations in population size, it does capture changes in long-term mean population density. In addition, the SPD reflects changes in productivity, and therefore reflects a changing population size.
Long-Term Population Growth
The long-term population growth rate can be traced through historical records, archeological proxies, and ethnographic data. The long-term patterns of population growth can be linked to evolutionary processes and various aspects of population ecology. For example, environmental stress, resource competition, and interference competition are all factors that may affect population size.
In the past, human population growth has been slow or non-existent. But since the last few centuries, it has grown rapidly. The curve resembles an exponential curve. Until the early nineteenth century, the world population grew at a rate of 0.04% annually. This growth rate has now slowed to one percent per year.
Evolution of Anatomically modern Homo Sapiens
The first remains of anatomically modern Homo sapians were discovered in Jebel Irhoud in Morocco, about 315 kya ago. This suggests that the species had already migrated across Africa at this time, and was well-adapted to its environment. Later, the species spread further south and eventually reached the southeastern fringe of its geographic range. This migration eventually led to the first humans’ settlement in Australia.
The earliest fossils of Homo sapiens are similar to those of early Neanderthals. However, the facial features of these animals differ greatly from those of anatomically modern Homo sapiens. The cranium is high and the face is small and retracted under the frontal bone. The face and pelvic shapes are also distinct in H. sapiens, and the ear of these animals is also distinct.
Archaic Homo Sapiens
Modern humans have different skulls and brains from archaic Homo sapiens. The braincase of modern humans is much smoother and globular compared to archaic hominins. The frontal lobe of modern humans is also larger than the lobes of archaic hominins. This change requires a larger skull to accommodate the larger frontal lobe. Some scholars argue that modern Homo sapiens evolved in Africa, while others argue that they spread across the Old World, replacing archaic humans.
Since Linnaeus’ time, a great fossil record has been discovered. This fossil record includes extinct species that were more closely related to modern humans and apes. Some paleoanthropologists extend the human ancestor’s range further into the distant past, while others attribute fossils to other extinct species.
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Early sapiens in Africa had a complex morphological history, and there is no clear linear progression from early sapiens to modern sapiens. However, fossils from the middle Pleistocene of Africa reveal an overlap between archaic and modern morphologies.