MOSCOW, May 17, Nikolai Guryanov. For billions of years, the Earth was inhabited exclusively by unicellular organisms. It is still unknown how and why more complex life forms arose. To answer this question, scientists reproduced evolution in the laboratory.
From «jelly» to «tree»
A team led by William Ratcliffe from the Georgia Institute of Technology (USA) has been conducting a decades-long multicellular evolution experiment (MuLTEE) since 2018. In May 2023, scientists released the first interim report.
The unicellular yeast Saccharomyces cerevisiae was taken as the basis. This type of mushroom is widely used for the manufacture of bakery products, alcoholic beverages, and also for scientific research. It was he who became the first nuclear organism (eukaryote), whose genome was completely deciphered — in 1996.
In the new experiment, the scientists grew baker's yeast in a test tube, selecting the largest and fastest growing on a daily basis. In about three thousand generations, an organism that could not be seen without a microscope has grown to the size of fruit flies. Growth — 20 thousand times.
Individual structures developed unexpected properties, becoming as strong and rigid as wood. To understand how this happened, the scientists «enlightened» multicellular clusters with a scanning electron microscope (SEM) and saw a section of growing cells.
«We found that there is a completely new physical mechanism that allows groups to grow to very, very large sizes , — says one of the authors of the work Ozan Bozdag. — The «branches» of the yeast became tangled — the cluster cells began to behave like a grapevine, twisting around each other and strengthening the entire structure. «
This was a turning point in understanding how simple multicellular groups evolve. Yeasts do not have complex developmental mechanisms characteristic of modern multicellular organisms. But after just 3,000 generations of laboratory evolution, they «figured out» how to manage cellular entanglement and use it for growth, the authors note.
Ratcliffe believes that a long experiment will allow us to trace the evolution of early life forms — up to the appearance of the so-called Darwinian entities capable of sustainable multicellular evolution.
External threat
This is not the first experiment on the transformation of unicellular organisms into multicellular ones. In 2021, a group of Professor Lutz Becks from the University of Konstanz (Germany) showed that single-celled green algae Chlamydomonas reinhardtii develop mutations in just 500 generations that provide the first step towards more complex life forms.
According to one version, multicellular organisms formed from a colony of single cells, which, after division, continued to hold together as a group. Becks and his colleagues subjected primitive algae to the pressure of natural selection by placing a predator, a rotifer, in a test tube. It turned out that she couldn't — or at least found it harder to eat cells that clustered together than those that stood alone.
In other «cups», algae were allowed to live without external threat. A follow-up of five hundred generations showed that colonies formed much more often, increased in size, and had a higher reproductive capacity in environments where the predator was active than where it was not.
Thus, the theory of the origin of multicellular life was experimentally confirmed — it occurs only when groups of cells reproduce better and have more chances of survival than individual ones, the authors of the work note.
— «Snowball Earth»
In 2017, Jochen Brox of the Australian National University in Canberra suggested that the emergence of multicellular life is associated with the global glaciation of the Earth. «At that time there was a real ecological revolution, a kind of revolt of algae,» the researcher said.
The first organisms appeared on Earth three billion years ago, but remained single-celled. More complex forms of life are thought to have evolved as little as 600-650 million years ago, during the Ediacaran period.
According to Brox's theory, the first photosynthetic multicellular algae appeared at that time. It is known that «light-eating» unicellular creatures — bacteria and archaea — inhabited the Earth before.
Studying the deposits of sedimentary rocks at the bottom of the Earth's primary ocean — now it is the central part of Australia — Brox and his colleagues discovered fat molecules, which are abundant in animal cells and plants, but not in cyanobacteria and archaea.
These rocks formed during the era when the Earth began to thaw after turning into a giant «snowball» about 850 million years ago. The primary ocean received a large amount of phosphorus and other nutrients from rocks mixed by the movement of glaciers. It was phosphorus that gave plankton an advantage over photosynthetic bacteria. The result was food webs that gave rise to ever larger and more complex organisms—and eventually life as we know it today.