Life on Earth: From Micro to Macro
This is a guest post by Dr. Asim Sahebzada, Ph.D. who is a geo-scientist and hails from Pakistan.
Why should we care in the first place as to how did the ancient life looked like or how have humans and other primates evolved? Humans were sent by God from Heavens is a good enough reason for many, however, there is a better reason to care about the processes that have sculpted human evolution. Understanding this question is the key to understanding why people look and behave the way they do. In this context, it is important to re-examine questions and expressions such as; why are we fighting like animals or even better, why are we fighting worse than animals? ISIS is worse than animals; why do people look so different around the world? What is the future of humanity? All of these questions and many more are related to the evolution of the human body, mind and behavior.
In an age of horrendous ethnic conflicts and increasing strain on respect for multicultural diversity, we are constantly reminded of the variations within human species. For this very reason, it is essential that we strive to understand how life evolved on Earth. Humans have always been baffled by their existence and they have looked for answers since the dawn of modern human civilization.
Darwin’s discovery of the principles of evolution by natural selection in 1838 has subsequently been augmented by discoveries in genetics and amplified by studies of the evolutionary patterns of thousands of land and aquatic organisms. The theory of Evolution, along with the theory of Plate Tectonics, now provides the foundation for our basic understanding of how life evolved on Earth.
While studying human evolution, it is important to note that life first appeared on Earth some 4 billion years ago (bya) in the form of unicellular organisms which most probably developed by chemosynthesis around hydrothermal vents or black smokers in paleo-oceans. Hydrothermal vents are hot chimneys which bring enormous amounts of minerals and metals from deeper horizons and deposit them on the ocean floors. The contribution of comets bringing huge amounts of water to the newly formed Earth must be appreciated and somewhere in that process they may have brought specs of life with them as well. After initial accretion at 4.6 bya, the Earth was still a molten ball of magma. Once the Earth cooled down significantly in the first 500 million years, continents started to form from nuclei of small islands. There is no evidence to suggest that any life form existed in the first 1.4 billion years of the Earth’s history.
The Earth’s history is organized in Geologic Time Scale and is divided into several categories called eons, eras, periods and epochs in descending order of time segments. The earliest period is called the PreCambrian (570 mya to 4.6 bya), followed by Paleozoic (570 mya to 280 mya), Mesozoic (280 mya to 65 mya) and finally Cenozoic spanning from 65 mya to the present time.
Cambrian Explosion took place around 570 mya, where unicellular organisms were converted into multicellular organisms, mostly invertebrates. Life remained exclusively in oceans for the next 150 million years. Trilobites were dominating the oceans while amphibians were exploring new territories on the fringes. Plants decided to invade the land around 400 mya and about the same time amphibians were giving way to reptiles. Paleozoic or ancient life came to an end in a mass extinction 280 mya when trilobites and 95% of marine life became extinct. This was the first time life on Earth suffered a setback, but every tragedy has a silver lining and in this case reptiles and dinosaurs started to thrive both on land and in oceans.
The interplay of continents and oceans has occurred many times in geologic history and around 250 million years ago (mya) they were all lumped together in the form of a super-continent called the Pangea. Imagine the heat in the interior of such a huge continental mass whereas the coastlines were soaked with rainfall and enormous waves! In this super-ocean, majority of the Earth’s life was thriving.
Continents were drifting apart, colliding with each other, moving away from each other, new oceans and mountain chains were forming and consequently new climates and life forms were evolving. Large mountains and oceans create impediments to movement of land life and consequently adaptations lead to the creation of new species. Madagascar (not the movie) was once part of India when it was drifting towards Eurasia but somehow it was left behind where it is now, hence it has its own unique fauna and flora. Continental Drift and Sea Floor Spreading (the two component theories of Plate Tectonics) are essential to understanding the fundamentals of Evolutionary process.
Mesozoic (or middle life) came to an abrupt end when an asteroid struck the Earth creating Chicxulub Crater, just west of Mexico’s Yucatan Peninsula. The intruding body was six miles across and unleashed the power of hundreds of thousands of atomic bombs. The effects were widespread from the Cretaceous-Tertiary (or K-T) mass extinction and one estimate is that there was complete blackout of the sunrays for at least six months and the skies remained murkey for at least 10 years. This basically killed all the photosynthetic activities on the Earth’s surface which are essential for living organisms. The increased volcanic activity around this time (e.g., creating the Deccan flats in India) made things even more complicated. Imagine the problems these events created for the living organisms including the big guys! Dinosaurs, owing to their large size, could not withstand the catastrophic events and most other organisms failed to adapt to the emerging climatic conditions.
Mammals remained insignificant throughout Mesozoic as dinosaurs straddled the top of the evolutionary pyramid. Once the giants were gone, stage was set for great mammalian evolution and spectacular radiation. Cenozoic (or recent life) is rightly called the mammalian era where thousands of mammal species are occupying various nitches both on land and in oceans. While discussing human evolution, we almost always forget the simultaneous evolution of plant species. Mesozoic was dominated by gymnosperms which resembled modern conifers, however early Cenozoic era saw the radiation of angiosperms (flowering plants) which bore colorful fruits. Savannahs and grasslands developed simultaneously, and combined with angiosperms, provided shelter, food and matting spaces for mammals, birds and insects which were not available in earlier time periods.
Placental and marsupial mammals that bore live young evolved at the beginning of Cenozoic and all of the modern descendents such as horses, bats, whales, lions, elephants and primates evolved and radiated with favorable climatic and ecological conditions. In order to understand human evolution from a small organism to the modern Homo sapiens, it is essential to comprehend that complex adaptations are assembled gradually, in numerous small steps that are favored by natural selection.
Humans evolved over millions of years and if you “believe” the Earth was formed 6000 years ago then there is no room for understanding all the intervening evolutionary steps. It will make perfect sense to you that the Grand Canyon was carved by Noah’s floods and dinosaurs roamed the earth along with humans! However, critical thinking and reasoning paint a different story; all organisms are related to each other and the fossil record is a testimony to that. The fossil record maybe incomplete but it points to discenrable patterns in biological evolution.
Evolution is a trial-and-error method; if an organism fails to adapt to the new environment, s/he is on its way out. Armadillos are a recent example of evolutionary failure; they have not updated their hardware and software, so to speak, and that’s why they are just about to become extinct (besides being killed on the roads). You cant install Windows10 on a PentiumII machine!
Bipedal locomotion, dentition, larger brains as a proportion of body size, flat nails as opposed to claws, long juvenile periods and communication through well developed languages are traits that separate humans from other primates (gorillas, orangutans, baboons, lemurs, chimpanzes, gibbons, macaques, monkeys and apes). These genetic and ecological adaptations are a product of mutations and natural selections and took millions of years to arrive at the current state.
Modern humans are complex organisms evolved over millions of years. They exhibit different traits based on genetics and environment in which they are present now and the traits of their ancestors. Humans also have inherent differences due to their socio-economic conditions and it is essential that all those differences are taken into account before any labels are applied. Humans are highly complex organisms and the more we study their origin, the better we will understand their differences and similarities. At the end of the day humans are evolved animals!