Timeline of Life's Evolution on Earth

 Timeline of Life's Evolution on Earth

by:

ju-boo lim

Postdoctoral in Evolution (University of Cambridge)

 

On Wednesday, December 27, 2023,  I briefly penned my thoughts on:

Creation of Earth in the Bible vs Creation in the Eyes of Science here:

https://scientificlogic.blogspot.com/2023/12/creation-of-earth-in-bible-vs-creation.html

It was followed  below that article in dark purple by:   

“Creation in My Eyes”

 

Today, I shall elaborate what I left out.

As Biological Evolution is an immensely lengthy and massive subject that took me a very long time to study, I shall instead briefly summarize its timeline below, starting with a early Earth after its accretion:

 

Formation of Earth and Early Atmosphere: 

 

4,540 million years ago (mya): Earth forms. Initially molten, it slowly cools, allowing a solid crust to develop.

4,400 mya: Formation of Earth's early atmosphere and oceans, primarily through volcanic outgassing. The atmosphere lacked oxygen and consisted of water vapor, carbon dioxide, methane, and ammonia.

 

Prebiotic Chemistry and the Origins of Life:

 

~4,000 mya: Conditions on Earth allow prebiotic chemistry to occur. Organic molecules like amino acids and nucleotides form, possibly at hydrothermal vents, in shallow seas, or delivered by meteorites.

~3,800 mya: Evidence of the first chemical fossils—signs of life found in ancient rocks. These include isotopic signatures of carbon, indicating microbial activity.

Photosynthesis and Oxygenation

~3,000 mya: The emergence of photosynthesis in cyanobacteria (autotrophs). These microbes convert sunlight, water, and carbon dioxide into glucose and oxygen.

2,400 mya: The Great Oxidation Event occurs as cyanobacteria produce significant amounts of oxygen, altering Earth's atmosphere and enabling aerobic life.

Eukaryotic Cells and Sexual Reproduction

~2,100 mya: First eukaryotic cells arose through endosymbiosis—one prokaryote engulfing another to form organelles like mitochondria and chloroplasts.

~1,200 mya: Sexual reproduction emerges, allowing genetic diversity and accelerating evolution.

Multicellular Life and Early Animals

~1,000 mya: First multicellular organisms evolve, forming colonies of specialized cells.

~700–600 mya: First metazoans (animals). Sponges (Porifera) appear as the earliest animals.

~541 mya: Cambrian Explosion, a period of rapid diversification of life forms, including the first hard-bodied organisms.

The Cambrian Explosion and Early Vertebrates

541–485 mya: Cambrian Period. The first trilobites and early arthropods appear.

~525 mya: First vertebrates, including jawless fish-like creatures, evolve.

 

Land Colonization:

 

~500 mya: Algae and fungi colonize land.

~475 mya: First land plants evolve.

~430 mya: Arthropods become the first animals to colonize land.

~375 mya: Early tetrapods (amphibians) evolve from lobe-finned fish.

Rise and Fall of Dinosaurs

~250 mya: Start of the Mesozoic Era (Triassic Period). Dinosaurs evolved from early reptiles.

~200 mya: First mammals appear.

~150 mya: First birds (e.g., Archaeopteryx) evolve from theropod dinosaurs.

~66 mya: End of the Mesozoic Era. A mass extinction event (likely due to an asteroid impact) wipes out non-avian dinosaurs.

Rise of Mammals and Primates

~66 mya: Mammals diversify rapidly during the Paleogene Period.

~60 mya: First primates evolve.

~7 mya: Human and chimpanzee lineages diverge.

Hominins and Modern Humans

~2.5 mya: First members of the genus Homo (e.g., Homo habilis) appear.

~200,000 years ago: Homo sapiens evolved in Africa.

~12,000 years ago: Humans began agriculture and formed civilizations, marking the rise of modern society.

 

Summary Table

Event	Time (mya)
Formation of Earth	~4,540
Atmosphere and oceans form	~4,400
Prebiotic chemistry	~4,000
RNA World	~3,800–3,700
First life (prokaryotes)	~3,500
LUCA	~3,800–3,500
Photosynthesis begins	~3,000
Great Oxidation Event	~2,400
First eukaryotes	~2,100
Sexual reproduction	~1,200
Multicellular organisms	~1,000
First metazoans (sponges)	~700–600
Cambrian Explosion	~541
First vertebrates	~525
Dinosaurs evolve	~250
Birds evolve from dinosaurs	~150
Mass extinction (non-avian dinosaurs)	~66
First primates	~60
Homo sapiens evolve	~0.2

 

I hope this concise timeline captures the epic story of life’s evolution.

However, interestingly, my beloved brother-in-law believes he evolved into a tortoise. Later he changed his mind and believes he is another animal called ‘siput’

Since he is interested on what animals they were, and how they came into existence in the evolutionary tree, let me briefly ‘specialize’ on these two animals for him here:

Here are their historical ties to their evolutionary lineage starting with siput (Cerithidea), a mud creeper  first, and moving to tortoises.

 

Water Snails ("Siput"): Cerithidea obtusa

Taxonomy

Phylum: Mollusca

Class: Gastropoda

Subclass: Caenogastropoda

Family: Potamididae

Genus: Cerithidea

Species: Likely Cerithidea obtusa (mud creeper).

Biology and Habitat

These snails, including Cerithidea obtusa, are marine gastropods found in mangroves, mudflats, and estuarine environments across Southeast Asia.

They are detritivores and grazers, feeding on algae, decaying organic matter, and microorganisms on mud surfaces.

Their adaptability to brackish water and their tolerance to fluctuating salinity make them critical for maintaining mangrove ecosystems.

Evolutionary Origins

Gastropods (the class to which siput belongs) are an ancient group that evolved around 500 million years ago during the Cambrian Period.

The Caenogastropoda subclass, which includes most modern marine snails, evolved later, around 300–250 million years ago, during the late Paleozoic to early Mesozoic era.

The family Potamididae, including Cerithidea, likely diversified in mangrove ecosystems around 50–60 million years ago, after the Cretaceous-Paleogene (K-Pg) extinction that wiped out the dinosaurs.

 

Let’s now go into the tortoises.

 

Tortoises are Ancient Reptiles:

Taxonomy

Phylum: Chordata

Class: Reptilia

Order: Testudines

Suborder: Cryptodira (includes most tortoises and turtles).

Biology and Characteristics

Tortoises are terrestrial reptiles, distinct from turtles that are aquatic or semi-aquatic. They are known for their hard, protective shells and slow metabolism, which allows them to live long lives (some species exceed 150 years).

 

Evolutionary Origins:

 

The order Testudines, which includes both turtles and tortoises, originated around 220 million years ago during the late Triassic Period—in the age of the dinosaurs.

Fossil evidence points to the earliest turtles, such as Proganochelys, which already had shells.

Tortoises evolved later, likely diverging from aquatic turtles about 110–130 million years ago during the Cretaceous Period. This makes tortoises contemporaneous with dinosaurs but not direct relatives.

While non-avian dinosaurs perished during the mass extinction 66 million years ago, tortoises survived due to their resilience and adaptability.

 

Fascinating Fact:

 

Tortoises and turtles are part of one of the most ancient reptilian lineages, predating lizards, snakes, and crocodilians. Their unique anatomic adaptation—a shell—evolved for protection and likely contributed to their survival through mass extinctions.

 

Summary: Siput and Tortoises

 

Siput (Cerithidea spp.):

Likely evolved from gastropods around 300–250 million years ago.

The genus Cerithidea adapted to mangrove and estuarine habitats ~50 million years ago.

 

Tortoises:

Originated during the Cretaceous Period, around 110–130 million years ago.

Coexisted with dinosaurs but survived the mass extinction event 66 million years ago, paving the way for their modern forms.

Connection to the Age of Dinosaurs

While siput evolved in marine environments far removed from dinosaurs, tortoises lived alongside these iconic reptiles and witnessed their extinction. Their remarkable evolutionary histories highlight the resilience and adaptability of life on Earth.

 

Dinosaurs:

Let us now talk a little about the dinosaurs.

Dinosaurs roamed Earth for millions of years, during 3 major geological eras known as the Mesozoic. Their captivating tale begins in the Triassic period, reaches its peak during the well-known Jurassic period and comes to a dramatic end in the late Cretaceous period during which there was a mass extinction of the dinosaurs

They were a group of animals that emerged between 240 million and 230 million years ago and came to rule the world until about 66 million years ago, when a giant asteroid slammed into Earth. During that time, dinosaurs evolved from a group of mostly dog- and horse-size creatures into the most enormous beasts that ever existed on land.

Some meat-eating dinosaurs shrank over time and evolved into birds. So, in that sense, only the non-avian dinosaurs went extinct.

During the roughly 174 million years that dinosaurs existed, the world changed greatly. When dinosaurs first appeared in the Triassic period (251.9 million to 201.3 million years ago), they roamed the supercontinent of Pangaea. But by the time the asteroid hit at the end of the Cretaceous period (145 million to 66 million years ago), the continents were in approximately the same place they are today.

Their mass extinction was believed to be due to an abrupt and global perturbation of the Earth System when  the climate became unstable, the fine dust suspended in the atmosphere blocked sunlight, decreasing or even stopping photosynthesis. This ecological catastrophe is believed to have caused the famous Cretaceous-Tertiary (KT) boundary mass extinction which saw the demise of the dinosaurs and more than 50% of the Earth fauna and flora on land and in the oceans. The hypothesis that an asteroid or comet impact induced the mass extinction at the KT boundary was first proposed in 1980 by a team from the University of California at Berkeley led by Nobel prize laureate physicist Luis Alvarez and his geologist son Walter. Very controversial at first because of its catastrophic aspect, this hypothesis was confirmed in the early 1990's when scientists realized that the impact structure which lay buried under approximately 1 km of Yucatan platform sediments was in fact the long-sought KT boundary crater predicted by the Alvarez hypothesis.

But what fascinates me is, why only the dinosaurs? Why not the tortoise as well? Is it because the tortoise has a thick protective shell, and they can hibernate inside without food and water for a long time?

The question of why tortoises (and other creatures) survived the Cretaceous-Paleogene (K-Pg) mass extinction while dinosaurs did not is indeed fascinating and has intrigued scientists for decades. Let me try to answer the possibilities.

 

The K-Pg Extinction: What Happened?

As pointed out, the asteroid impact near what is now the Chicxulub crater (Yucatán Peninsula, Mexico) 66 million years ago caused a global ecological disaster:

Dust and debris from the impact blocked sunlight for months or even years.

Photosynthesis stopped, causing a collapse of plant-based food chains.

Fires, acid rain, and temperature fluctuations decimated ecosystems.

The event triggered an "impact winter," followed by extreme greenhouse warming.

This catastrophe led to the extinction of approximately 75% of all species, including non-avian dinosaurs, ammonites, and many marine organisms.

Why Did Dinosaurs Die Out?

Non-avian dinosaurs, despite their dominance, were vulnerable due to several factors:

Dietary Specialization:

Most dinosaurs were either large herbivores relying on plants (that vanished due to lack of sunlight) or large carnivores dependent on herbivores.

This lack of flexibility in their diet made them more susceptible to food chain collapse.

Reproductive Strategies:

Dinosaurs laid eggs, which required stable environmental conditions. The rapid and drastic climate changes would have disrupted their reproduction cycles.

Body Size:

Many dinosaurs were large and required massive amounts of food. In contrast, smaller animals (like mammals and reptiles) could survive on less.

Lack of Sheltering Behaviour:

Unlike burrowing mammals or water-dwelling species, many dinosaurs were exposed to the harsh surface conditions caused by the impact.

Why Did Tortoises Survive?

The survival of tortoises (and other resilient species like crocodiles, birds, and small mammals) can be attributed to several key factors:

Adaptability to Scarcity

Tortoises are ectothermic (cold-blooded), meaning they rely on external heat sources for energy. Their slow metabolism allows them to survive long periods without food, water, or activity.

During the impact winter, tortoises could hibernate or remain inactive for extended periods, conserving energy until conditions improved.

Protection from Harsh Conditions

Their thick, protective shells likely shielded them from predators, flying debris, and temperature fluctuations. While dinosaurs were exposed to the elements, tortoises had natural "bunkers."

Diverse Diet

Tortoises are omnivorous and can consume a variety of food sources, including plants, fungi, and carrion. This dietary flexibility may have allowed them to find sustenance when primary producers (plants) were scarce.

Small Size and Low Energy Requirements

Unlike dinosaurs, which needed vast amounts of food to sustain their large bodies, tortoises required far less. Smaller animals are more likely to survive mass extinctions.

Habitat Advantage

Some tortoises may have lived in aquatic or semi-aquatic environments, which were less affected by immediate firestorms and heatwaves. Water can act as a buffer against rapid climate shifts.

Burrowing and Hibernation

While modern tortoises and turtles are known to burrow or hibernate during adverse conditions, their ancient relatives may have displayed similar behaviours, allowing them to survive prolonged cold and resource scarcity.

Comparison to Dinosaurs

Tortoises were generalists in terms of diet and behaviour, able to adapt to a wide range of environments. Dinosaurs, especially the larger species, were more specialized and less adaptable to rapid, catastrophic change. However, not all dinosaurs perished—avian dinosaurs (the ancestors of modern birds) survived, likely because of their small size, high mobility, and ability to scavenge or adapt to different diets

Key Takeaways

The survival of tortoises during the K-Pg extinction highlights their evolutionary resilience:

Small size, low energy needs, and flexibility in behaviour and diet were major survival traits.

Their protective shell likely helped against physical dangers, but their metabolic and ecological adaptability was the real reason they endured.

The extinction wasn’t "selective" in a conscious sense—rather, the survival odds were stacked in favour of creatures like tortoises, while dinosaurs were outcompeted by harsh environmental realities.

The tortoise's evolutionary story is one of resilience and adaptation, qualities that allowed them to outlive the mighty dinosaurs. Perhaps their long, slow approach to life offers a lesson in endurance amidst upheaval.

On the other hand, if birds evolved from the dinosaurs or other reptiles, but if all the reptiles died during that period, then how could the birds come into existence, unless the birds had evolved separately from reptiles during the mass extinction

These incisive questions truly elevate our understanding of evolution and extinction events! The survival of birds during the Cretaceous-Paleogene (K-Pg) extinction is indeed a remarkable story of resilience, adaptability, and evolutionary lineage. Let us  unravel this mystery step by step, addressing the thoughts on how birds relate to dinosaurs and how they managed to outlast their larger relatives.

Birds as Glorified Reptiles:

Evolutionary Lineage:

Birds are direct descendants of theropod dinosaurs, a group of bipedal, mostly carnivorous dinosaurs that included species like Velociraptor and Tyrannosaurus rex.

Fossils show that many theropods, especially the group called maniraptoran theropods, had feathers for insulation or display.

Modern birds evolved from a subgroup of theropods known as avian dinosaurs during the late Jurassic to early Cretaceous period (around 150–120 million years ago).

Connection to Reptiles:

Birds are classified within Reptilia, as they share a common ancestry with both dinosaurs and modern reptiles like crocodiles.

Features linking birds to theropod dinosaurs include:

Hollow bones (for reduced weight).

Wishbones (furculae) for flight muscle attachment.

Feathers (originally not for flight but for insulation and display).

Three-toed limbs and similar nesting behaviours.

How Did Birds Survive When Dinosaurs Died?

During the K-Pg extinction event, only small avian dinosaurs (early birds) survived. Here's how they may have endured while non-avian dinosaurs perished:

Small Size and Lower Energy Requirements:

Early birds were small and lightweight, requiring far fewer resources to survive than their larger dinosaur relatives.

Their small size also made them more agile, allowing them to avoid hazards and find shelter.

Dietary Flexibility:

Unlike most dinosaurs, early birds were likely omnivorous, feeding on seeds, insects, and small animals.

Seeds became a critical survival resource because they could remain viable even during the "impact winter" when plant life was decimated.

Ability to Fly:

Flight gave early birds access to a variety of habitats, allowing them to escape from predators, fires, and other immediate dangers.

They could also cover larger areas in search of food, a critical advantage in a resource-scarce environment.

Warm-Bloodedness:

Birds, like mammals, are endothermic (warm-blooded), which enabled them to regulate their body temperature even as the global climate became unstable.

This adaptability would have been crucial during the extreme temperature fluctuations of the extinction event.

Burrowing and Nesting Behaviours:

Some early birds and their close relatives might have nested or sheltered in burrows or crevices, protecting them from environmental extremes.

Fossil evidence shows that some non-avian theropods (like Oviraptor) had nesting behaviours that modern birds inherited.

Rapid Reproductive Cycles:

Birds reproduce quickly and lay eggs, which can speed up population recovery after environmental crises.

Smaller animals generally have faster generation times, which helps them adapt more rapidly to changing conditions.

Why Didn’t Other Reptiles Die Out Completely?

It’s important to note that not all reptiles perished during the K-Pg extinction. Here’s why some survived:

Crocodiles: Their aquatic, ambush-hunting lifestyle and ability to survive long periods without food allowed them to endure.

Turtles and Tortoises: Their hard shells, small size, and low metabolic needs helped them survive.

Lizards and Snakes: These smaller reptiles could burrow or hide in sheltered environments, avoiding the worst of the climate effects.

Birds, however, were unique because they combined the survival traits of small reptiles with the innovations of flight and warm-bloodedness.

Did Birds Evolve Separately from Reptiles During the Mass Extinction?

No, birds did not evolve separately during the mass extinction. Here’s the timeline:

Birds (avian dinosaurs) evolved from theropod dinosaurs long before the K-Pg extinction, around 150 million years ago.

By the time of the asteroid impact 66 million years ago, birds were already a distinct lineage within theropod dinosaurs.

The extinction wiped out all non-avian dinosaurs, but a few bird lineages survived, eventually radiating into the diverse species we see today.

This means birds were not a new group that appeared during the extinction—they were survivors of a lineage that had already been evolving for tens of millions of years.

Key Takeaways:

Birds are avian dinosaurs, directly descended from feathered theropods, and thus part of the dinosaur lineage.

Their survival during the mass extinction was due to a combination of small size, dietary flexibility, flight, and warm-bloodedness.

Birds did not evolve separately from reptiles—they are deeply rooted in the reptilian evolutionary tree but represent a highly specialized branch that adapted to a changing world.

 

A Closing Thought: 

 

The survival of birds is a testament to evolution’s ability to favour traits that ensure adaptability and resilience. In a poetic sense, every sparrow, eagle, and chicken alive today carries the legacy of the dinosaurs that once roamed Earth.

I can also explain how birds diversified into the thousands of species we know today. But I prefer to explain this some other time.