It is believed that the earliest forms of life were very simple single-celled organisms but
new research is uncovering that these very simple life forms form into complex colonies
that are far more animal-like than we ever realised. Let's dive in and find out more.
We have previously looked at how single-celled organisms were able to store collective
memories and use this information to make decisions in order to find food sources.
We know that at some stage in our past single cells started to function together forming
multicellular entities. Previously I examined an experiment which revealed that slime mold,
a single-celled organism, was able to store collective memories
and use this information to make decisions in order to find new food sources.
Bacterial cells band together in clumps to form tightly packed colonies called biofilms.
These biofilms should not be seen as slimy blobs of cells,
instead, new research is revealing that they are far more sophisticated than first thought.
A biofilm is made up of a collection of microorganisms
in which cells stick to each other and often to a surface. Microbial cells growing in a
biofilm are physiologically distinct from the planktonic cells of the same organism,
which by contrast are single cells that float or swim in a liquid medium.
Microbes form a biofilm in response to a number of different factors.
These can include cellular recognition of specific or not-specific attachments sites on the surface,
nutritional cues, or in some cases, in response to a threat.
A cell that switches to the biofilm mode of growth undergoes a change in both morphology
and physiological behaviour in which large suites of genes are differentially regulated.
It is believed that biofilms arose during primitive Earth as a defense mechanism
for those early cells.
Scientists are starting to realise that these biofilms are far more sophisticated than they
may at first appear. Many resemble how plants and animals develop segments as they grow.
New research is also revealing that these biofilms
are able to share collective memory similar to neurons in the brain.
The biofilms also seem to be capable of recruiting other bacterial species
to join their communities using long-range electrical signals.
As they continued to study them they observed bacterial biofilms forming ring-like structures
that are remarkably similar to the developmental stripes often seen in plants and animals.
This type of cellular patterning is how multicellular organisms
form different types of tissues and complex body forms.
It was believed that single-celled organisms could only form the most primitive of structures.
Their research demonstrates that bacterial biofilms employ a developmental patterning
mechanism previously thought to be exclusive to vertebrates and plant systems.
When they observed Bacillus subtilis, a rod-shaped bacterium that is found in soil and humans
and forms wrinkly biofilms, they deprived it of nitrogen and something remarkable
happened. The growing biofilms organised themselves into clear circular bands,
resembling tree rings and the sort of segmentation you see in developing embryos.
The researchers believe that this ring-like pattern
is created by an underlying genetic circuitry in the bacterial cells.
These are triggered in response to extreme stress when nutrients such as vital nitrogen are limited.
The experiments revealed that as the biofilm grew outwards consuming nutrients
a wave of nutrient depletion moved across the bacterial cells. These
acted to freeze each cell in place with the stress mitigating genes.
As the stress response pulsed through the entire colony, creating on and off stress
responses it created repeating segments of different cell types in the circular biofilm.
In a different piece of research, scientists were able to demonstrate how biofilm growth
mirrors embryonic development, with the expanding colonies following a highly organised sequence
of gene expressions over several months. This led the leading scientist to state that:
Considering that the oldest known fossils are bacterial biofilms, it is quite likely that
the first life was also multicellular, and not a single-celled creature as considered so far.
These findings are raising questions about what defines a multicellular organism
when the simple single-celled organisms appear to be far more advanced than we first thought.
But how different are single-celled organisms to multi-cellular organisms and how quickly can then
evolve? This is a question that a different group of scientists explored. Researchers
from the Georgia Institute of Technology ran an experiment over a 50 week period. They took simple
single-celled algae and introduced a predator into the experiment and observed what happened.
Incredibly in just 50 weeks, two out of five experimental populations of single-celled
organisms evolved into multicellular life. This represents just 750 generations for the algae
which is a very short timeframe. The resulting
multicellular organisms were all incredibly varied.