Welcome to milestones in evolution.

Abiogenesis is the technical word for the origin of life. We believe this happened on Earth (as opposed to another planet – this was an idea that held sway in the 1970s, but is not considered as likely today). Abiogenesis probably happened before 3,800 million years ago: exactly when, how, and what happened next, are the contents of this part of the course.


We're going to cover:

  • The origins of life and abiogenesis – Section 1 - 2.
  • Early evolution, archaea and bacteria – Section 3.
  • The great oxygenation event – Section 4.
  • The origin of eukaryotes & sexual reproduction – Section 5.
  • The origins of multicellularity, and ediacaran organisms – Section 6.
  • Animal origins, and early evolution – Section 7.

Throughout these materials: Ga = 1000 million years, Ma = 1 million years.

This is important stuff. Understanding how life life may have originated on Earth is key to understanding it more generally, and the likelihood of there being life elsewhere in the universe. Then those key steps on the way to where we are today are also foundational to numerous fields. For example, life has fundamentally altered Earth's atmosphere. Cancer is a disease that occurs when the controls on multicellularity don't work as intended. So as well as this all being fascinating, what we cover has broad implications.

1 – What is life?

In order to discuss the origins of life, first we have to define life. This video covers definitions of life and when the origins of life (abiogenesis) may have occurred.


  • Defining life is a challenge. In general we can say living organisms:
    • Maintain themselves
    • And replicate... Imperfectly.
  • Abiogenesis, on the balance of probabilities, probably occurred between 4.4 and 4.0 Ga.

Something to consider

Following on from the last question, you might like to think about whether you consider a virus to be alive or not. This is in part semantic, but it also digs down into what we really mean by life. I've provided a link to further reading in the bonus material which isn't obligatory, but is worth checking out if you are interested in this topic.

2 – Abiogenesis!

Another key thing to consider in abiogenesis is where the materials came from that are required for life as we know it. In this video we look at the source of these materials, and then how abiogenesis itself may have occurred.


  • Material for life probably accumulated through:
    • Input from asteroids, comets and other extraterrestrial sources.
    • And earth-bound synthesis.
  • There are two primary theories for abiogenesis, which aren't necessarily (in my view) mutually exclusive:
    • Prebiotic soup theories – largely cold and oceanic, informational molecules early.
    • Metabolist theories – hot and associated with deep sea vents, metabolism before informational molecules.

A question to think about

How likely do you think abiogenesis was to have occurred? Was it inevitable? Highly unlikely?

3 – Whence life as we know it?

Here we look at the earliest steps that may have occurred in the evolution of life on Earth – just after its origins.


  • Life's current informational molecules are RNA and DNA, but there were probably simpler precursors.
  • All life shares a common ancestor – LUCA.
  • There is a fundamental split in tree of life between two groups of prokaryotes - the archaea and bacteria.
  • The earliest well-corroborated microfossils appear at 3.43 Ga.
  • You can find the tree of life I mentioned in this video at this location.

Before we get onto the next video please take a moment to think about the following question, and then participate in the poll!

4 – The spread

Now we advance to the first widespread evidence of life on earth, the evolution of photosynthesis and great oxygenation event, and look at some lovely microfossils from 1,800 million years ago.


Life appears to then become widespread on earth fairly quickly, and has a huge impact on the chemistry of the Earth system.

  • Stromatolites – sedimentary structures associated with life – become widespread by 3 Ga.
  • Free oxygen in the atmosphere originates through photosynthesis, accumulated primarily at ~2.5 Ga – the great oxygenation event (GOE).
  • The appearance of O2 marks the end of the Archaean and start of the Proterozoic Eon.


Below you can find some 3D models of stromatolites – please do take the time to look at these (you may want to close one before opening the next so only one is open at any time). They are actually quite common if you do geology fieldwork, and are the first macroscopic evidence of life on earth!

Fossil specimen of a stromatolite from the Silurian of Herkimer County, New York. Specimen is on display at the Museum of the Earth, Ithaca, New York. Model by Emily Hauf.

Fossil specimen of the stromatolite Collenia versiformis from the Proterozoic of Montana. Specimen is from the Cornell University Paleobotanical Collection (CUPC), Ithaca, New York.

Fossil specimen of the stromatolite Chlorellopsis coloniata from the Eocene of Wyoming. Specimen is from the Cornell University Paleobotanical Collection (CUPC), Ithaca, New York. Model by Emily Hauf.

5 – Eukaryogenesis

Here we enter the super interesting world of more complex cells: those of Eukaryotes. We find out how this group originated, and look at one particular part of the eukaryote cell, the mitochondrion, which allows you to respire oxygen.


  • Eukaryotes arose through a process called endosymbiosis, which is an extreme form of symbiotic (i.e. mutually beneficial) behaviour.
  • Eukaryote cells include membrane-bound sub-structures called organelles.
  • Eukaryotes are more closely related to archaea than bacteria.
  • The origins of the nucleus is a matter of highly active research.
  • Organelles such as mitochondria / chloroplasts have clearer origins: via endosymbiosis of bacteria.
  • Eukaryogenesis (the origin of the group) had probably occurred by ~1.5Ga.
  • Sexual reproduction creates a significant cost for eukaryotes. But it must produce a selective advantage – perhaps due to pathogens.

6 – Macroscopic forms

Some Eukaryotes have become multicellular! What does this mean, and how did that happen? This video has a clip in it from BBC studio's First Life in it – included through the university's ERA Licencing scheme – but I have hosted it on Manchester servers on this basis.


Multicellularity is hard to define – but a rough working definition for our needs is multiple cells forming an individual and those cells specialising to different functions. We can say:

  • That multicellularity is seen throughout the eukaryote tree of life.
  • It originates in the majority of these instances through symbiosis of organisms of the same species to form a colony.
  • The earliest definitive evidence of cellular differentiation, and sexual reproduction, is found in Bangiomorpha pubescens – a 1.2 Ga red algae.
  • Ediacaran organisms, that appear at 635 Ma are the first macroscopic, multicellular organisms to be widespread in the fossil record. Where they sit on the tree of life is challenging to deduce.

7 – Animals!

We finish this series of videos by looking at the origins of animals. How do we define animals, and when did they evolve? Lets delve into the Cambrian explosion.


  • Animals appear suddenly in the fossil record at ~540Ma.
  • We are still research the extent to which this reflects a genuine burst of evolution, as opposed to a change in – for example – fossilisation potential.
  • What drove this event remains a matter of highly active research.

Bonus stuff!

That was lots of complicated stuff! Well done for sticking with it. I think it's really interesting, and if you agree – and would like to learn more – I've put some little bonus bits you can use to delve a little deeper below.

Quiz results!

First, I figured that you may be interested in the result of the question at the end of the last quiz! The question was, are viruses living? The response, so far, is shown below.

If you would like to explore this question a little more this article from the Microbiology Society is a good place to start, and has some links to further reading at the end. One thing is clear what has happened over the last six months: dead or alive, understanding viral evolution and viruses is integral to keeping societies safe, economies running, and countries' borders open.

The absolute latest in Ediacaran Palaeobiology

I've placed a video below of a talk by my hugely talented colleague Dr Frankie Dunn, given as part of a lecture series by the Oxford Museum of Natural History (all of which are well worth your time!). As well as introducing the very latest research on frond-like Ediacaran species, it will also stand you in good stead for the forthcoming lectures on evolutionary relationships.


  • Rangeomorphs are Ediacaran taxa probably lie on the stem lineage to much of animal diversity.
  • But they had body plans that would not have been predicted by looking at just living creatures, making them hard to interpret.
  • A key tool is looking at the growth and development of these fossils.

If you like this, and found it useful, there are more talks in this series that are highly relevant! You can find these here. That by Professor Paul Smith titled The Cambrian Explosion and the evolutionary origin of animals gives an insight into the Cambrian explosion, and the video The First Animals: When, Where and How? digs down further into this question.