How Did Complex Life Evolve?
Complex cells (eukaryotes) likely evolved from single-celled organisms (prokaryotes) absorbing other prokaryotes, becoming single complex cells over time (endosymbiosis). This theory helps to explain how cells good at producing energy from sunlight teamed up with cells good at surviving an oxygen rich environment to not only survive, but to thrive and evolve on earth. 
This endsymbiotic theory (AKA symbiogenesis), when combined with the theory of Darwinian evolution, the theory of how viruses affect the genetic code, and the general theory of the evolution of life on earth (see videos below), shows a clear theory of evolution from chemical reactions, to single-celled prokaryotes, to complex eukaryota-based life.
Endosymbiosis describes not only the origin of mammals, but of all cell based life including plants. This means evidence suggests endosymbiosis occurred multiple times with different types of organisms, specifically with mitochondria and plastids (for example chloroplasts).
TIP: Remember, this is theory and not “fact” (it is a model with facts pointing to it being true), if you need a refresher on what that means see our explanation of theories.
A TED-Ed video discussing how complex cells evolved through endosymbiosis.
See the links below for more additional reading, see the videos below for a walk through the history of life from the initial conditions on earth, to the first prokaryotes, to endosymbiosis, or keep reading to learn more about endosymbiosis.
- See Scientific American for how viruses may have affected evolution.
- Learn more about endosymbiotic theory.
- Learn more about Darwinian evolution.
A Brief History of Life: When Life Part 1. This video walks you through the initial conditions of earth all the way up to endosymbiosis.
A Brief History of Life: When Life Part 2. This video discusses evolution after endosymbiosis.
The Evolution of Complex Cells Simplified (Endosymbiotic Theory Simplified)
Billions of years ago on earth single-celled organism A “absorbs” single-celled organism B, and they join to become a single complex-celled AB organism. The relationship is beneficial to both with each performing an essential function for the other. Complex-celled organism AB passes on its new form for generations creating many AB organisms.
Today we don’t find A or B organisms, but we find lots of AB organisms and we think we’ve found ancestors of A and B respectively too. All human, plant, and animal life contains these complex AB cells. This strongly suggests that all life evolved from the joining of A and B organisms billions of years ago.
FACT:The idea that single-celled prokaryotes merged with each other and became a more complex eukaryotes over time is based on a longstanding theory called symbiogenesis or endosymbiotic theory. The process of this happening can be referred to as endosymbiosis. There is significant evidence to suggests that single-celled organisms evolved into complex organisms in this way, but it’s still technically a “theory”. (NOTE: A theory can be true and is often regarded as fact).
Endosymbiotic Theory Terms Defined
To understand endosymbiotic theory in detail we need to define a few terms:
- Symbiosis occurs when two different species benefit from living and working together.
- Endosymbiosis refers to the symbiosis in which one of the symbiotic organisms lives inside the other.
- Endosymbiotic theory is a theory, based of evidence, that suggests that early prokaryotes (bacteria or archaea) evolved into a single more complex organism (eukaryote) through endosymbiosis.
- Secondary endosymbiosis is when one eukaryote absorbs another eukaryote.
- Prokaryota (Prokaryotes) are single-celled organism that lacks a cell nucleus, mitochondria, or any other membrane-bound organelle.
- Archaea and Bacteria are both prokaryotes with slightly different genetic features. They may or may not have the same origin. Often you’ll hear the prokaryotes discussed in endosymbiotic theory as bacteria, but today we think some or all may have actually been archaea. We’ve only known about archaea since the 1990’s but have been discussing the theory of symbiogenesis since the early 1900’s.
- Eukaryota (Eukaryotes) are complex cells that contain mitochondria. Mitochondria preform “cellular respiration” in a symbiotic relationship with the host cell. Plant eukaryota also contain chloroplasts, which have a similar function to mitochondria.
- Mitochondria are double membrane-bound organelles found in most eukaryotic cells. They contain a short loop of DNA that is distinct from the DNA contained in a eukaryote cell’s nucleus. When inside the Eukaryote, they perform cellular respiration for the cell generating most of the cell’s supply of “energy” or adenosine triphosphate (ATP).
- Chloroplasts are similar to mitochondria and are found in plants. They use photosynthesis to generate the cell’s ATP.
- An organelle is essentially an organ of a cell. It’s a subunit in a cell that preforms a specific function. Mitochondria and chloroplasts function as organelles for the eukaryote, they are the primary way cells generate energy making them vital to the eukaryote and all life on earth.
NOTE: Some refer to the original prokaryotes that came that came together through endosymbiosis as bacteria, others think mitochondria and chloroplasts are actually archaea which differs slightly from bacteria and is considered to be part of a different group.
Endosymbiotic Theory Summary
Endosymbiotic theory describes how a large host cell prokaryote and its ingested prokaryote could easily become dependent on one another for survival (as they each performed a unique and mutually beneficial function), resulting in a permanent symbiotic relationship.
The First Eukaryota
We don’t know how life emerged on earth (although it’s speculated that it occurred as a chemical reaction or came from archaea, which can survive in space and other extreme conditions). While we don’t know exactly how life got here, we do have evidence that suggests life first emerged at least 3.8 billion years ago, approximately 750 million years after Earth was formed. During this time there were two types of organisms are though to have existed, single-celled prokaryotes called bacteria and archaea.
We think that around 1.5 billion years ago two of original prokaryotes, mitochondria and chloroplasts (or at least mitochondria at first), joined with a third prokaryote creating the first eukaryote.
The new hybrid prokaryotes (now eukaryotes) actually had an evolutionary advantage as they could perform the functions of both prokaryotes for the single eukaryote. This advantage was passed on to generations of new eukaryotes.
Friends With Benefits
It is thought that the absorbing cell had a permeable membrane and protective cytoplasm surrounding its nucleus. The absorbed cells (mitochondria and chloroplasts) enjoyed the protection and potentially the essential nutrients from the host’s cytoplasm. In return the host used the their talent for converting energy to ensure a readily available food source when its usual food source was depleted.
Different models exist that differ in explaining when the first prokaryote became eukaryote and the order in which mitochondria and chloroplasts became organelles of the two types of eukaryota. In essence the specifics of how and when things occurred exactly is debated, but the underlying concept is generally accepted as fact.
The Evidence of Endosymbiosis
Evidence of endosymbiosis includes:
- Mitochondria are found in almost all eukaryote cells. Eukaryote cells are found in all life. (Plants uniquely also have chloroplasts).
- Mitochondria and chloroplast “organelles” have their own DNA, which is separate from the DNA found in the nucleus of the eukaryote cell. (Although it is thought that the organelles have transferred some of their own DNA to the host cell’s nucleus during their evolution).
- Mitochondria and chloroplasts also have a double membrane that seems to have come from their original single-celled host.
- Mitochondria and chloroplasts can only form through “splitting apart” (binary fission) while inside eukaryote. Eukaryotes can’t form more mitochondria or chloroplasts if they are removed from the cell.
- See Wikipedia’s evidence of endosymbiosis for more evidence.
Essentially we can see that these things that now function as organs for a cell have their own unique DNA and other features that strongly suggest they were once all separate prokaryotes.
Over millions of years of evolution, mitochondria and chloroplasts (both prokaryotes and likely archaea) have become more specialized and today they cannot live outside of eukaryotes. We don’t find mitochondria and chloroplasts outside of eukaryotes today, but we think we have found their ancestors living in plentiful numbers in the ocean.
Archaea or Bacteria?
Today, a group of single-celled organisms called archaea still thrive in extreme habitats (and more hospitable ones too). We think these are the ancestors of mitochondria and chloroplasts. Due to the ability of archaea to thrive in extreme conditions it is speculated that the first archaea may have come from space.
There is a third type of cell that is sort of a hybrid Eukaryote / Prokaryote. There is no consensus on how to classify it, it contains other cells but only has a single layer nuclear membrane. This may hold some answers in regards to the evolution of cells.
But It’s Just a Theory…
In its common meaning one may take the word theory to mean “speculation”, but this isn’t a correct interpretation in the field of science. Something being a theory does not imply that it is not true.