How Life Evolved
The General Scientific Consensus on How Life First Evolved
And Why It Matters in the Garden
Earth is believed to have formed approximately 4.54 billion years ago. In its earliest history, the planet existed in a molten state. Over the following 600 million years, gradual cooling allowed the surface to solidify, forming a stable crust by around 3.9 billion years ago, from which the earliest land masses emerged.
As the young Earth cooled, water vapour in the atmosphere condensed and fell as rain, forming the primordial oceans. It was within these ancient oceans that complex chemical reactions gave rise to the first living systems. Most scientists agree that life began in water, and this aquatic origin remains directly relevant to modern soils, which still function as water-based chemical systems supporting life.
By around 3.4 billion years ago, there is strong evidence for organisms resembling modern bacteria. At this stage, Earth’s atmosphere contained little or no free oxygen. These early microorganisms lived in conditions very different from those we experience today, yet they formed the foundations of all subsequent life.
Oxygen, Microbes, and the Living Soil
Around 2.4 billion years ago, photosynthesising cyanobacteria (often called blue-green algae) evolved. These microorganisms used sunlight to convert water and carbon dioxide into energy, releasing oxygen as a waste product. This gradual release of oxygen, known as the Great Oxidation Event, fundamentally altered Earth’s atmosphere and made complex life possible.
By approximately 2 billion years ago, oxygen levels had reached about 1%, fluctuating over long periods before rising to near-modern levels of around 21% by 800 million years ago. Without this microbial activity, neither plants nor animals—including humans—could exist.
In garden soils today, bacteria and cyanobacteria still perform many of these ancient roles. They drive nutrient cycling, contribute to soil structure, and support plant growth by making minerals biologically available. When gardeners protect soil life, they are supporting processes that began billions of years ago.
The Emergence of Complex Life and Ecosystems
The earliest known multicellular organisms appeared around 570 million years ago, as oxygen levels rose sufficiently to support greater biological complexity. Fossil evidence for these early visible life forms was first identified in rocks from Wales during the 18th and 19th centuries and dated to the Cambrian Period, named after Cambria, the Latin name for Wales. More recent studies suggest a refined date of around 543 million years ago.
Charles Darwin recognised that these complex organisms could not have appeared suddenly. He proposed that they must have been preceded by simpler, soft-bodied life forms that did not fossilise well. He was correct. Modern research has confirmed that vast ecosystems of microscopic life existed long before visible plants and animals emerged.
In gardening terms, this mirrors what happens beneath our feet. Healthy soil ecosystems are dominated not by large organisms, but by microscopic life—bacteria, fungi, protozoa, and archaea—that collectively regulate nutrient flow, plant health, and ecosystem stability.
Life Before the Cambrian and the Soil Food Web
Advances in microscopy have revealed that the Precambrian period was rich in life, although invisible to early fossil hunters. These early organisms were mostly unicellular prokaryotes, similar in structure to modern bacteria and cyanobacteria. Microfossil evidence suggests that life may have existed as early as 4.0 to 3.7 billion years ago.
This immense span of time, known as the Proterozoic Eon, lasted nearly two billion years and saw relatively little change in overall organism structure. During this time, the basic biochemical systems that still operate in soils today became established. Nucleic acids, amino acids, and metabolic pathways that evolved billions of years ago remain central to life in modern ecosystems.
In a wildlife-friendly garden, the soil food web reflects this ancient lineage. Bacteria decompose organic matter, fungi form vast underground networks connecting plant roots, protozoa regulate microbial populations, and nutrients are continually recycled. These interactions are not modern innovations, but deeply rooted evolutionary processes.
From Deep Time to the Garden Gate
Understanding how life first evolved helps place gardening within a much broader ecological context. Soil is not an inert growing medium; it is a living system shaped by billions of years of evolution. Every act of mulching, composting, avoiding chemical disturbance, or encouraging plant diversity supports processes that pre-date flowering plants, insects, and even land itself.
Throughout this e-book, biodiversity is presented not as a collection of individual species, but as a network of relationships. By gardening in ways that protect soil structure, retain organic matter, and support microbial life, we align our practices with the natural systems that have sustained life on Earth for billions of years.
In doing so, the garden becomes more than a cultivated space—it becomes a small but meaningful continuation of Earth’s long ecological history.
From Deep Time to Garden Soil
How Billions of Years of Evolution Shape the Living Garden
When we look at soil in the garden, it is easy to think of it simply as a growing medium—a mixture of earth, compost, and organic matter that supports plants. In reality, soil is one of the most complex and ancient ecosystems on Earth. Its structure, chemistry, and living communities are the result of evolutionary processes that began billions of years ago.
The first life on Earth evolved in water, not on land. These early microorganisms—simple bacteria and cyanobacteria—developed the basic biochemical processes that still underpin all ecosystems today. The ability to recycle nutrients, capture energy, and interact chemically with the environment began long before plants colonised land. Modern soil life is a direct descendant of these ancient systems.
As oxygen gradually accumulated in Earth’s atmosphere through microbial photosynthesis, new forms of life became possible. Eventually, plants evolved the ability to live on land, forming partnerships with fungi that helped them access water and minerals from the developing soils. These early plant–fungal relationships are still present today in the form of mycorrhizal networks, which connect plant roots underground and allow the exchange of nutrients, water, and chemical signals.
Soil, therefore, is not simply broken rock mixed with organic debris. It is a living interface between geology and biology, shaped by microorganisms, fungi, plants, animals, air, and water over immense periods of time. Every teaspoon of healthy garden soil contains billions of bacteria, metres of fungal hyphae, and countless other organisms working together to sustain life above ground.
The Soil Food Web: An Ancient System at Work
The interactions between soil organisms form what is known as the soil food web. This system mirrors the earliest ecosystems on Earth, where life was dominated by microscopic organisms recycling nutrients and energy.
Bacteria break down simple organic compounds and make nutrients available to plants.
Fungi decompose tougher materials such as lignin and wood, while forming symbiotic relationships with plant roots.
Protozoa and nematodes feed on bacteria and fungi, releasing nutrients in plant-available forms.
Invertebrates, such as worms and insects, mix organic matter into the soil, improve structure, and create air and water channels.
These interactions are not modern developments. They represent evolutionary strategies refined over billions of years. When soil is disturbed, compacted, sterilised, or chemically overloaded, these ancient systems are disrupted.
Gardening as Participation in Deep Time
Every gardening decision influences whether these long-established processes are supported or suppressed. Practices such as mulching, composting, avoiding unnecessary digging, and reducing chemical inputs help maintain the integrity of soil ecosystems.
From an ecological perspective, wildlife-friendly gardening is not about controlling nature, but about working with systems that already exist. By allowing leaf litter to decompose, leaving roots in the ground after harvesting, and encouraging plant diversity, gardeners create conditions that resemble natural ecosystems more closely than heavily managed landscapes.
In this sense, the garden becomes a meeting point between deep evolutionary time and present-day human activity. The same microbial processes that once shaped Earth’s atmosphere now determine whether a garden soil remains fertile, resilient, and alive.
A Consistent Theme Throughout This E-Book
Throughout this e-book, ecology is viewed as a network of relationships rather than a collection of individual species. Soil life sits at the centre of this network. Healthy soils support healthy plants, which in turn support insects, birds, mammals, and the wider ecosystem.
By understanding soil as a living system shaped by billions of years of evolution, gardeners can make informed choices that enhance biodiversity rather than diminish it. Even small gardens, when managed with ecological awareness, can contribute to the protection and restoration of life-supporting processes that began long before humans existed.
In caring for garden soil, we are not creating something new—we are continuing a story that started deep in Earth’s past.