First DNA Life: From chemical reactions before 4 billion years ago to replicating molecules about 4 billion years ago to DNA based life then to LUCA, the only DNA based branch to survive. Meaning, DNA based life likely started before LUCA, and LUCA is the only branch to survive the test of time. From LUCA to humans, RNA and DNA are essentially the same as today. They serve as the tools for genetic information storage, transmission, and expression through prokaryotes to eukaryotes, underpinning the diversity and complexity of life on Earth. The last universal common ancestor (LUCA) is estimated to have lived approximately 3.5 to 3.8 billion years ago. It is the organism from which all current life on Earth descended from. Your greatest grandparents.

Imagined image: LUCA in a variety of shapes that they might have resembled, set in an ancient, deep-sea hydrothermal vent environment.

The earliest known life on Earth are fossilized microorganisms found in hydrothermal vent precipitates. Currently dated to about 3.42 billion BCE. These microorganisms were prokaryote cells. Single celled organisms with no nucleus and had early simple DNA. More complex DNA in a nucleus evolved about 1.5 billion years later in Eukaryotic cells, circa 2 billion BCE.

The Dawn of Photosynthesis and the Oxygenation of Earth: Around 2.4 billion years ago cyanobacteria emerged, the architects of the planet’s first photosynthetic processes. These microscopic prokaryotes harnessed the Sun’s energy, transforming it along with water and carbon dioxide into glucose and, crucially, oxygen. This period, known as the Great Oxidation Event, marked a dramatic increase in atmospheric oxygen levels, fundamentally altering the course of life’s evolution. Before this event, Earth’s atmosphere was largely devoid of oxygen, dominated instead by methane, ammonia, and other gases. 

Most life on Earth today consumes the Sun’s energy directly or indirectly, but not all life. Plants consume the Sun’s energy directly, plant eaters indirectly, and meat eaters one more step away. In addition to this chain of food, some life on Earth do not consume the Sun’s energy at all. Instead they consume nutrients found in extreme envorinments. Food to them like hydrogen sulfide.

Eukaryote cells evolve into three separate lineages, the ancestors of modern plants, fungi and animals. Later animals evolve into the animal kingdom which includes mammals, birds, reptiles, amphibians, fish, insects, crustaceans, arachnids, echiniderms, worms, mollusks, and sponges.

Long before the complexity of full-fledged nervous systems, elaborate senses, and brains, life on Earth developed the basic ability to perceive and react to mechanical stimuli—a process known as cellular mechanosensitivity. A contemporary manifestation of this can be observed in the movement of some plants today, such as the Venus flytrap, which responds to touch by snapping shut to capture prey. This early form of proto-sensing, relying on signal transduction pathways within single-celled organisms, represents the precursor to the sophisticated sensory and nervous systems found in later multicellular life. Through mechanisms like ion channel activation and signal transduction cascades, these primitive organisms could respond to touch and pressure, paving the way for the evolutionary journey towards more advanced forms of perception. This foundational stage of sensory evolution, occurring as early as 800 million years ago, underscores the deep biological roots of our ability to sense and interact with the world around us.

The evolution of egg-laying evolved before fish. It is thought to have developed with or shortly after the emergence of the first multicellular animals, around 640 million years ago.

Chemoreception, the ability to detect chemical stimuli, likely emerged around 600 million years ago among some of the earliest soft-bodied multicellular organisms. This evolutionary leap did not necessarily require a proto- or pre-brain in the complex sense associated with later animals but rather relied on cellular mechanisms capable of processing chemical information. These early forms of chemoreception enabled organisms to make rudimentary distinctions necessary for survival, such as identifying nutritive substances versus harmful ones. They could “taste” potential food sources upon direct contact and “smell” chemicals dissolved in the water, guiding them towards nourishment or away from danger. These primitive sensory mechanisms laid the groundwork for the sophisticated development of taste and smell in more complex animals.

Kimberella (circa 560 Mya): The Kimberella, while not an ancestor of vertebrates, is a likely early example of a creature with chemoreception. It is potentially an early mollusk, and exemplifies the importance of chemoreception in early animal life. Its grazing on microbial mats would have necessitated a basic form of chemoreception to discern between nutritious and non-nutritious substances. The distinction between food sources implies an elementary ability to ‘taste’ and ‘smell,’ integral for selecting suitable food. This behavior marks a significant step in the evolutionary sophistication of sensory systems, foreshadowing the complex senses of taste and smell found in later species.

Vision evolved as early as 540 million years ago during the Cambrian explosion. The ability to see, alongside the development of hearing, provided organisms with the evolutionary advantage of sensing their environment from a distance. This sensory evolution necessitated the development of larger brains for the complex processing of visual data, marking a pivotal moment in the cognitive evolution of life.

Pikaia Gracilens (520 to 505 Mya): In the ancient seas of the Middle Cambrian, starting around 520 million years ago, Pikaia gracilens swam into the annals of evolutionary history as one of the earliest known chordates, a group that would eventually give rise to vertebrates, including fish and, much later, humans. Measuring up to 6 centimeters in length, Pikaia boasted a series of notable features for its time, including a notochord—a flexible rod running along its back, which would become the backbone in its vertebrate descendants—and rudimentary structures suggesting the early development of a circulatory and nervous system. While Pikaia itself lacked well-defined eyes, its place in the evolutionary lineage hints at the beginnings of the complex sensory organs that would become eyes in later vertebrates.

The emergence of the first brains were likely in the earliest known hunters. By about 520 million years ago, hunters roamed the seas. In the Cambrian explosion, a period of rapid evolutionary development that began around 541 million years ago, the earliest known animals with structures recognizable as brains made their debut in the Earth’s oceans. They possessed rudimentary beginnings central nervous systems, including a brain. This allowed for advanced sensory processing, decision-making, and coordinated movement.

Anomalocaris (518 to 500 Million Years Ago): Among these early pioneers, creatures like Anomalocaris canadensis stand out. During the Cambrian Period, the Anomalocaris was a formidable predator of the seas. It reached lengths of up to three feet. With its large, compound eyes, flexible, segmented body, and a pair of grasping appendages in front of its mouth, it was perfectly adapted to detect and capture prey. Anomalocaris swam the ancient oceans with undulating movements, using its circular mouth lined with serrated plates to consume trilobites and other early marine animals.

Nectocaris pteryx lived during the Middle Cambrian period, approximately 508 to 505 million years ago. From presentient animals branched cephalopods and fish. Both later evolved Simple Sentience. An example of convergent evolution that might suggest sentience is one of the natural stepping stones of life.

The Cambrian and subsequent periods saw the emergence of early cephalopods, ancestors to modern octopuses, squids, and cuttlefish. These ancient cephalopods, navigating the Cambrian seas, possessed a more developed nervous system compared to many contemporaneous organisms, capable of processing information from their environment in sophisticated ways. This evolutionary development marked a significant leap towards simple sentience, with early cephalopods able to exhibit behaviors such as hunting strategies, escaping predators, and possibly even social interactions. The evolution of these early cephalopods highlights a pivotal moment in the history of life, demonstrating the beginnings of nervous system sophistication that would eventually lead to the complex forms of sentience observed in higher animals, including humans.

These early scales provided a vital protective layer, acting like underwater armor against predators, abrasions, and environmental threats. The keratin genes that led to scales are an interesting part of our story. When amphibians evolved onto land, scales near the tips of their evolving toes transformed into thicker protection and traction, eventually giving rise to claws, nails, and hooves. Meanwhile, cold temperatures and sun exposure led to the development of warming and protective traits like feathers, fur, and hair. Specialized “scales” evolved into horns in various species, such as triceratops and rhinos, despite their unrelated lineage. Additionally, keratin genes influenced the formation of beaks in birds. All these diverse traits originated from the same keratin genes.

The Dawn of Lungs: In the oxygen-poor waters of the Devonian period, roughly 400 million years ago, or a bit earlier. It was a significant evolutionary leap. A group known as Sarcopterygii were presentd with a formidable challenge in ancient waters that were shallow and variable. It was here that the first lungs emerged. Evolving from structures akin to modern fish’s swim bladders, these early lungs enabled them to extract oxygen directly from the air. This innovation marked a pivotal moment , setting the stage for land vertebrates.

The Legacy of the Lungfish: Descended from them, the lungfish epitomize the resilience and adaptability of life. Presently represented by six species across Africa, South America, and Australia, lungfish possess both gills and well-developed lungs, enabling them to survive in environments that would be inhospitable to other fish. During dry seasons, some lungfish can aestivate in mud, breathing air through their lungs until water returns. 

Hearing, which initially appeared in early fish, underwent a remarkable transformation as vertebrates transitioned to terrestrial life a bit after 400 million years ago. Early forms of hearing involved simple pressure-sensitive cells that could detect vibrations in water. As amphibians moved onto land, rudimentary hearing evolved into processing airborne sound. This transition further drove the need for enhanced brain capacity to process increasingly complex sensory information.

Acanthostega (circa 365 Mya): Hearing, a critical evolutionary advancement, underwent significant changes as vertebrates transitioned from aquatic to terrestrial environments. While the earliest forms of hearing evolved in water, allowing organisms to detect vibrations through their bodies, the move onto land posed new challenges and opportunities for auditory systems. Acanthostega, an early amphibian, exemplifies this transition. This period marks a crucial phase in the development of auditory systems capable of detecting a broader range of sounds, setting the stage for the complex hearing abilities observed in later terrestrial vertebrates.

Both reptiles and our ancestor synapsids evolved from amphibians. While reptiles evolved better amniotic eggs, synapsid eggs were like amphibian eggs. Synapsid’s birthing process eventually led to mammalian live births. These are the animals that evolved Complex Sentience, the ability to feel various emotions. While it is unknown when this complex spectrum fully evolved, it is defined as the ability to suffer and feel the dichotomy of pleasure and pain. Dimetrodon is an example of a meat eater, which if any of our ancestors were meat eaters. Although not a While dimetrodons were not direct ancestor of mammals, our mammalian ancestors might have been similar to our direct-line ancestors around this time. 

Class: Synapsids (pre-mammal, not a dinosaur, not pre-dinosaur)
Time Period: Late Triassic to Early Jurassic; 295-272 Mya; Diet: Carnivore

Animation of the break-up of the supercontinent Pangaea and the subsequent drift of its constituents, from the Early Triassic to recent (250 Million BCE to 1 CE). The super continent Pangaea existed from about 335 to 175 Million BCE.

An example of early live birth is the protomammal Kayentatherium, Jurassic period. This cynodont is related to early mammals and its clutch size suggested egg-laying, providing clues about the transition to live birth. The switch to live birth in mammals, including marsupials and placentals, likely evolved once at their common ancestor, suggesting live birth in mammals has a deep evolutionary history.

The appendix is a small, finger-shaped pouch attached to the large intestine. It has long been considered a vestigial organ, meaning that it has no function in the human body. However, recent research suggests that the appendix may actually serve as a reservoir for beneficial gut bacteria.

The appendix is an example of a Phenotype Variation — a trait that varies among individuals. In fact, something like 1 in 100,000 people are born without an appendix. The presence or absence of the appendix is one example of a variation in humans. However, the presence or absence of the appendix is not a typical example of phenotype variation, as it is not a continuous range of variation within a population. Nonetheless, it is an interesting variation that our descendants over the next eons will certainly observe.

Plesiadapis, a proto-primate, is an example of a fruit-insect eater likely similar to our direct-line ancestors around this time.

Class: Mammal; Early Primate (Plesiadapiformes)
Time Period: Late Paleocene
Diet: Likely Frugivorous/Insectivorous

Emerging in the lush forests of the Eocene, Miacis signifies a pivotal moment in the evolution of cognitive abilities among mammals. As a basal member of the Carnivora, this small, tree-dwelling creature exhibited behaviors and social dynamics suggesting the early stages of self-awareness. Though not akin to the self-recognition seen in humans or other highly intelligent animals, the life of Miacis and its interactions with its environment and conspecifics likely involved a level of awareness and individual recognition. These early forms of cognitive complexity mark the beginning of the path toward the rich inner lives characterized by self-awareness in later mammals.

Early Primate; Early Ape (hominoid): Proconsul, an inhabitant of the Miocene forests, stands as a landmark in the evolutionary journey toward self-awareness. This early ape lacked a tail and exhibited a mixture of arboreal and terrestrial traits, providing clues to the social and environmental challenges that likely spurred cognitive advancements. While direct evidence of self-awareness in Proconsul is beyond our reach, its position in the ape lineage suggests the development of social structures and cognitive abilities that predate the sophisticated self-awareness observed in modern great apes, elephants, and dolphins. Proconsul’s world was one of increasing cognitive complexity, setting the stage for the emergence of true self-aware beings, capable of recognizing themselves as distinct entities within their social and natural environments.

Following the divergence of the orangutans, the gorilla lineage branched off from the rest of the great ape family around 8 to 9 million years ago. This split led to the evolution of the largest of the living apes, the gorillas. They exhibit a fascinating social structure and exhibit impressive displays of strength, yet are known for their gentle nature. The divergence of gorillas from the common ancestor they shared with humans, chimpanzees, and bonobos further highlights the rich tapestry of evolution that has given rise to the variety of great ape species seen today.

Location: Emerged in Western or Central Africa; spread to Eastern Africa and no further.
Size: Up to 5’8″ and 550 pounds (Eastern Lowland Gorillas)