New York, Jan 26 (IANS): How did cells make the transition from life as a solo cell to associating with other cells to become a single, cohesive unit?

The answer lies in plants which have changed in size, shape, structure and reproduction over the past few million years, says a new study.

Karl Niklas, a plant evolutionary biologist at Cornell University in New York, reviewed the history of multicellularity.

He looked at the changes that cells must have had to go through - such as aspects of their shape, function, structure and development - in order to be able to functionally combine with other cells.

Later, he explored the underlying driving forces and constraints - from natural selection to genetics and physical laws - that influence the evolution of multicellularity.

"Multicellularity is a fundamental evolutionary achievement that is capable of mathematical description and one that has occurred multiple times in different plant lineages," Niklas said.

Scientists agree that multicellularity has occurred multiple times across many clades - a group that includes a common ancestor and all the descendants (living and extinct) of that ancestor.

Multicellularity, or aggregation of cells, has evolved once in animals, three times in fungi, six times in algae and multiple times in bacteria, said the article that is part of a series of centennial review papers celebrating 100 years of the American Journal of Botany.

"The evolution of multicellular organisms occurred multiple times and involved different developmental 'motifs' - such as the chemistry of the 'glues' that allow cells to stick together," Niklas said.

However, there are certain sets of requirements that must be met in order for multicellularity to evolve.

These include that cells must adhere to, communicate with, and cooperate with each other, and that cells must specialise in their functions, said the study.

In order to make these things happen, cells must not reject each other.

In other words, they must be genetically compatible to some extent - analogous to how our human bodies reject foreign items that are not recognised by our cells.

"The evolution of multicellular organisms is like the old saying: there are many roads to Rome, but Rome is not what it used to be," summerised Niklas.