Common Origin of All Living Things

Table of Contents

What “Common Origin” Means

When biologists say that all living things share a “common origin,” they mean that:

Every organism on Earth—bacteria, plants, fungi, animals, humans—descends from a single original population of organisms (often called the last universal common ancestor, LUCA).
Differences between life forms arose gradually over immense time through evolutionary changes, not as separate, unrelated beginnings.

This chapter focuses on the evidence that all organisms are related, not yet on the detailed fossil record or mechanisms of evolution (those are covered in other sections).

Universal Features of Cells

All known organisms (except some borderline entities like viruses) are made of cells. Despite enormous diversity, cells share several basic features that strongly suggest a shared origin.

Shared Basic Architecture

In both prokaryotic and eukaryotic cells:

Plasma membrane:

All cells are surrounded by a membrane built from a phospholipid bilayer.
This structure, with hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails, is universal.

Cytoplasm:

All cells contain a watery interior with dissolved ions, small molecules, and macromolecules.
Basic processes—like glycolysis (a core pathway of energy metabolism)—occur in this interior in all organisms.

Ribosomes:

All cells use ribosomes to synthesize proteins.
Prokaryotic and eukaryotic ribosomes differ in detail, but their core structure and function are conserved, indicating descent with modification from an ancestral ribosome type.

Universal Energy Currency and Core Metabolisms

ATP (adenosine triphosphate):

Every cell uses ATP as a universal energy carrier.
Energy from nutrients or light is converted into ATP, which then powers cellular work.
This is highly unlikely to have arisen independently multiple times in exactly the same way.

Core metabolic pathways:

Glycolysis (breakdown of glucose):

Present in almost all organisms, from bacteria to humans.

Parts of the citric acid cycle and aspects of electron transport are also widely shared.
The same or very similar enzymes catalyze the same reactions across very different organisms, suggesting inheritance from an ancestral metabolic network.

The Universal Genetic System

The strongest evidence for a common origin lies in how organisms store, copy, and use genetic information.

DNA as the Genetic Material

In all cellular organisms:

Genetic information is stored primarily in DNA.
DNA has the same double-helix structure built from four nucleotides: A, T, G, C.

Differences between species lie in the sequence of these nucleotides, not in the basic chemistry.

The Universal Genetic Code

Proteins are made of 20 standard amino acids. The instructions for assembling them are written in DNA/RNA using triplets of bases called codons. Across almost all life:

The same codon system is used. For example:

AUG codes for the amino acid methionine and often serves as a start signal.
UAA, UAG, and UGA serve as stop codons, signaling the end of protein synthesis.

This pattern (the genetic code) is nearly identical in bacteria, plants, animals, and fungi.

There are only minor variations in the code (e.g., in some mitochondria or certain protists), and these can be explained as small, later changes from an originally universal code.

The probability that completely separate origins of life would independently choose:

DNA as genetic material,
RNA as an intermediate,
the same four nucleotides,
the same triplet code mapping to the same 20 amino acids,
is astronomically low. A single origin with later divergence is a far more plausible explanation.

Same Information Flow: DNA → RNA → Protein

In virtually all cellular life, genetic information flows in the same general pattern:

$$
\text{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}
$$

Transcription: DNA serves as a template to make RNA.
Translation: Ribosomes read RNA codons to assemble amino acids into proteins.

Although there are special cases (e.g. some viruses reverse the direction using reverse transcriptase), the basic scheme is shared across life, again indicating a common plan inherited from an ancestor.

Molecular and Genetic Similarities

Beyond the general architecture of the genetic system, detailed molecular comparisons show deep relatedness.

Conserved Genes and Proteins

Some genes are found in all or nearly all organisms, often with very similar sequences. Examples include:

Genes encoding parts of ribosomal RNA (rRNA).
Genes for key enzymes in basic metabolism (e.g. parts of glycolysis, ATP synthesis).

These “housekeeping” genes are so conserved that:

A human version and a bacterial version often share recognizable sequence similarities.
They perform essentially the same function despite hundreds of millions (or billions) of years of divergence.

This pattern is what you expect if all organisms inherited these genes from a shared ancestor, with gradual mutation and selection over time.

Phylogenetic Trees from Molecular Data

Biologists can compare DNA or protein sequences from different species and create phylogenetic trees—branching diagrams that represent evolutionary relationships.

Key observations:

Independent genes (e.g. from ribosomes, metabolic enzymes, etc.) often produce similar tree patterns, grouping organisms in the same way.
These molecular trees frequently agree with relationship patterns inferred from anatomy, development, and fossil evidence.

The simplest explanation is that these trees reflect true shared ancestry, not chance similarity.

Molecular Clocks

Some genes accumulate mutations at roughly regular rates. Using such genes:

Scientists can estimate how long ago two lineages shared a common ancestor.
When applied across a wide range of organisms, these estimates suggest:

All cellular life diverged from a common ancestor more than 3 billion years ago.

These independent timing estimates are broadly consistent with geological and fossil evidence for early life.

Universal Biochemistry

All life uses the same basic chemistry:

Same fundamental building blocks:

Amino acids for proteins (largely the same 20).
Nucleotides for nucleic acids.
Sugars (e.g. glucose) and related carbohydrates.
Phospholipids for membranes.

Same chiral preference:

Biological amino acids (except glycine) are almost all L-forms.
Biological sugars used in nucleic acids and metabolism are mostly D-forms.
Random independent origins would likely produce mixtures of left- and right-handed forms, but life shows a consistent global bias, as if all descended from a single “choice” made early on.

Shared coenzymes and carriers:

Many organisms use the same molecules such as NAD⁺/NADH, FAD/FADH₂, Coenzyme A, etc.
These carry electrons or chemical groups in countless reactions across very different species.

This shared chemical toolkit is most naturally explained by inheritance from a common ancestor whose biochemical solutions were so successful that they were retained and elaborated upon.

Hierarchical Patterns of Similarity

When comparing living organisms, we see nested sets of similarities:

All vertebrates share characteristics not found in other animals (e.g. backbone).
Within vertebrates, mammals share extra traits (hair, mammary glands).
Within mammals, primates share yet more traits (grasping hands, certain skull features).
Within primates, great apes share still more traits, and so on.

The same occurs at molecular levels:

Some proteins are found in all life.
More specialized proteins are found in only certain major groups.
Highly specialized features are limited to very narrow groups.

This nested, hierarchical organization is exactly what you would expect if:

There was a single ancestral population.
New traits evolved and were inherited by all descendants of that population.
Over time, lineages continued to split, each adding its own innovations.

If different organisms had appeared independently, you would not expect such a consistent, nested pattern of shared derived traits.

Convergence vs. Common Origin

Some similarities between organisms arise because of similar environmental pressures, not shared ancestry (this is convergent evolution—covered in detail elsewhere). However:

Convergence typically affects specific traits (e.g. wings, body shape) and often uses different underlying structures.
The evidence for common origin comes mainly from:

Deep, basic features (cell structure, genetic code, core biochemistry) that are shared by all life.
Extensive molecular similarities across the entire genome.

Convergence cannot reasonably explain the near-universal genetic code, shared metabolic pathways, and conserved housekeeping genes. Those are best explained by descent from a single ancestral system.

LUCA: The Last Universal Common Ancestor

The last universal common ancestor (LUCA) is:

Not the very first life form ever, but the most recent organism (or population) from which all currently living organisms descend.
Reconstructed indirectly by:

Identifying genes and molecular systems shared by all life.
Inferring which traits LUCA must have had to give rise to present diversity.

Based on this, LUCA:

Likely already had:

DNA as genetic material.
RNA, ribosomes, and the genetic code.
Many core metabolic pathways.
A cell membrane.

Probably lived in an environment rich in chemical energy, possibly near hydrothermal vents or other energy-rich habitats.

The concept of LUCA ties together the universal features of life as traits inherited from a common starting point.

Summary of Key Lines of Evidence

Evidence that all living things share a common origin includes:

Universal cell architecture (membranes, ribosomes, cytoplasm).
Shared core metabolism and ATP-based energy transfer.
Nearly universal genetic code and information flow (DNA → RNA → protein).
Conserved genes and proteins with recognizable sequence similarities across all domains of life.
Molecular phylogenetic trees and molecular clocks that converge on a single ancestral lineage billions of years ago.
A common biochemical toolkit (same building blocks, same coenzymes, same chiral preferences).
Hierarchical patterns of shared traits consistent with branching descent.

Taken together, these independent lines of evidence strongly support the conclusion that all living organisms on Earth are part of one extended family, tracing back to a single ancient common ancestor.

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