We think of human evolution as a natural, step-by-step feature of life on Earth, even if all evidence suggests that it’s not the case. Over the eons, life on Earth had to go through many different, unique steps to lead to the evolution of modern humans, including our decision to stand up on two feet instead of four and move from hunter-gatherer societies to settled, farming communities.
10. Last Universal Common Ancestor
LUCA refers to the Last Universal Common Ancestor – a theoretical ancestor to all life forms that exist today. Existing some time around four billion years ago, it would have played a pivotal role in the evolution of life on Earth.
According to simulations, it would have lived deep underground in iron-sulfur rich hydrothermal vents, possibly in dark, metal-rich environments. It would have also been anaerobic and autotrophic, not relying on oxygen and producing its own food from hydrogen, carbon dioxide, and nitrogen.
According to many scientists working on tracing the genetic tree of life, LUCA would be the root of the tree and the beginning of life’s evolutionary journey. Sadly, LUCA’s exact physical organization and properties remain theoretical and – to a large extent – imaginary. While the traditional understanding of evolutionary sciences depict LUCA as the last common ancestor of archaea, bacteria, and eukaryotes – the three main branches of life forms that exist today – our understanding of the subject has also undergone significant changes due to new data and analytical techniques in the past few years, further muddying our understanding of the evolution of early life.
9. The Great Oxidation
The Great Oxygenation Event happened some time around 2.7 billion years ago, when the planet’s atmosphere had no free oxygen and life primarily consisted of anaerobic organisms that didn’t rely on it to survive. The emergence of a group of microbes called cyanobacteria, however, profoundly changed that equation, as this was when they developed the ability to carry out photosynthesis and produce oxygen as a waste product. As they thrived and released huge amounts of oxygen into the oceans, it gradually accumulated in the atmosphere and laid the basis for most modern life today.
This sudden rise of free oxygen had far-reaching consequences for life on Earth. While it became vital for the evolution of aerobic organisms, the oxygen proved to be toxic for the anaerobic cyanobacteria and many other anaerobic life forms, leading to a mass extinction that wiped out over 90% of life on Earth. A few life forms managed to adapt and utilize the newly-available free oxygen, leading to the evolution of aerobic metabolism. The Great Oxygenation Event was a turning point in Earth’s history that completely altered the planet’s chemistry, paving the way for diverse, oxygen-based organisms we see everywhere around us today.
8. Evolution Of Complex Cells – Eukaryotes
The evolution of complex cells, or eukaryotes, was a major milestone in the history of life on Earth. Prokaryotes, or simple cells like bacteria, emerged around four billion years ago as some of the most ancient life forms. Eukaryotes, however, do not appear in the fossil record until over a billion years later, around 2.7 billion years ago. The origin of eukaryotic cells still remains a mystery, though we know that it was a turning point for the evolution of all complex life found around the world today.
The primary difference between eukaryotic and prokaryotic cells lies in their complexity. Cells found in eukaryotic organisms contain a nucleus, specialized organs for energy production like mitochondria, and a cytoskeleton that provides structural support and allows for the movement and growth of complex cells. While their origin still remains a scientific mystery, they directly gave rise to all modern multicellular organisms, including humans.
The emergence of multicellularity was another important step during our evolution, as multicellular organisms display many properties that arise from communication between cells and their environment. Many recent studies have proved the importance of these dynamic interactions in various biological processes found across the tree of life today, including collective cell migration, cell differentiation, and cell proliferation.
Throughout our history, multicellularity has emerged out of single-celled organisms at different times, even if we don’t quite understand how it happened. The capacity to form multiple cells and specialized cell types, in turn, has resulted in the evolution of diverse organisms, including plants, animals, algae, and fungi. Multicellularity has also had a huge impact on Earth’s biodiversity and ecological complexity, as without it, our planet would lack the diversity of life forms we observe today.
6. Cambrian Explosion
The Cambrian Explosion refers to the emergence of a surprisingly-high number of organisms around the planet about 541 to 530 million years ago. Many major classes of animals that make up modern animal life showed up during this period, leading to the diversification of life forms. The evolution of hard body parts like calcium carbonate shells allowed for better-preserved fossil specimens to study and analyze, leading to a better understanding of this time than any other period in the early history of life on Earth.
While there are still some doubts about exactly what caused the Cambrian explosion, we know that increased oxygen levels in the atmosphere around this time likely played a role in this unprecedented proliferation of complex animals across diverse types of ecosystems on Earth. The rapid appearance of various marine animals during this time profoundly impacted Earth’s biodiversity, setting the stage for further expansions in the coming periods.
5. The First Sea Animals Move To Land
The first marine animals started moving to land more than 470 million years ago, when plants colonized the mainland during the Ordovician period. Their presence fundamentally altered land ecosystems, as it oxygenated the atmosphere, shaped the soil for future plant evolution, and established new climatic conditions that could house the oncoming diversity of animals.
While organisms like arthropods, myriapods, and arachnids came to land during the Silurian period around 430 million years ago, insects – or hexapods – followed around the beginning of the Devonian period around 410 million years ago. This transition from marine to terrestrial life was a major turning point in the early history of the planet, leading to the evolution of life on Earth as we know it today.
The ability to walk entirely on two feet – or bipedalism – represents a distinct evolutionary milestone in human evolution, setting us apart from other four-legged apes. This adaptation is marked in our evolutionary history by several changes in the spine, pelvis, skull, and feet, enabling hominids to stand upright and walk on two legs. However, all this came at a high cost, like overloaded lower backs and lower extremity joints that now cause knee and back pain later in life, especially for females during childbirth.
Initially, scientists believed that it was big brains that distinguish hominids from other species. The discovery of the Taung Child’s skull by Raymond Dart in the 1920s, however, greatly changed that belief. Subsequent fossil findings further supported the idea that bipedalism preceded the development of larger brains in human evolution.
3. The Rise Of Homo Sapiens
Homo sapiens evolved in Africa around 300,000 years ago during a period of climate change. Like other early human species at that time, we adapted to survive in unstable environments by gathering and hunting for food. Anatomically, modern humans can be characterized by our lighter skeletal build compared to earlier humans, large brains, and distinct facial features with less prominent brow ridges.
Prehistoric Homo sapiens developed advanced stone tools, specialized them, and made more refined and complex tools like bows, arrows, and fishing equipment. Over time, we transitioned from hunting and gathering to farming and herding, leading to the transformation of Earth’s landscapes, settlements, and a marked rise in the human population.
While early Homo sapiens coexisted with other human species like Homo erectus, the Denisovans, and Neanderthals, that changed around 40,000 years ago, when we emerged as the last surviving member of the hominin family.
2. Leaving Africa
The first humans moved out of Africa some time around 2 million years ago, though the challenging arid environments of northern Africa and Middle East initially hindered their movement. To leave Africa successfully, our ancestors had to develop physical and mental capabilities suitable for surviving in harsh conditions, where food and water were scarce and seasonal.
The first species believed to have left Africa was Homo ergaster – or African Homo erectus – about 1.75 million years ago. An alternate theory, however, suggests that hominins might have migrated out long before that, possibly around 2 million years ago. Recent studies based on DNA analysis support it, indicating an expansion out of Africa about 1.9 million years ago, with gene flow between Asian and African populations around 1.5 million years ago.
Regardless of the timeline, the ability to leave Africa allowed us to develop physical attributes like efficient walking, intelligence for adapting to new environments, improved tools, and a diet that included more meat, all of which would play a crucial role in human evolution for years to come.
1. Neolithic Revolution
The rapid development of agriculture in human populations and proliferation of agriculture-based societies around the world – also known as the Neolithic Revolution – was perhaps the most decisive step in the evolution of modern humans. This pivotal transition happened some time around 10,000 BC in the Fertile Crescent region in the Middle East, leading to the emergence of the first farming communities we know of.
The Neolithic Revolution marked a shift from small, nomadic hunter-gatherer groups to larger settlements and the birth of early civilizations. It was a time of profound and long-lasting change for our species, as this was when we first started cultivating plants and breeding animals for food, along with forming permanent communities to protect those advancements.
The causes of the Neolithic Revolution are multifaceted and may have varied across regions. One theory is that changes in the climate and a warming trend in the Middle East around 14,000 years ago played a role in the development of agriculture there, leading to the growth of wild wheat and barley in the Fertile Crescent. Intellectual advances in human societies – including the development of religious and artistic practices – also likely influenced the transition to settled farming.