A new study in cellular biology has shed more light on the evolutionary history of life on planet Earth, starting with an explanation of how multicellular organisms developed from unicellular ones, over a billion years ago.
The story so far has life breaking out on Earth (through a process still under much debate) about 3.8 billion years ago in the form of single-celled organisms such as bacteria. Some two billion years would then go by before multicellular life takes shape and eventually leads to the emergence of plants, animals and everything else. Two billion years is quite a timespan, but the jump from uni- to multicellular was momentous. While unicellular organisms can be highly successful (that yeast in your craft beer seems to be doing alright), there are advantages to having different cells in the same organism working towards the same goal: survival.
But how did this leap occur, exactly? What process moved the single cell to associate and take up residence with another cell and why would this have transpired? Theories abound. One has it that during the process of division (one form of single-celled reproduction) some single-celled creatures failed to divide properly, subsequently formed membranes around the doubled-up cell nuclei and, voila! Two-celled organism! Another popular theory holds that some unicellular organisms developed the advantage of cooperation with each other, to the point where they became inseparable and their genetic codes became linked, thus creating a single multicellular being.
But a new study published in the journal Developmental Cell proposes something else, namely, that perhaps the jump from one- to many-celled was less about newfound cooperation between cells or mistakes in division than about single-celled organisms developing internal mechanisms similar to those found in multicellular life forms, genetic mechanisms, for instance, which regulate internal cell processes over different stages in a cell’s life.
Researchers studying a single-celled amoeba called Capsaspora owczarzaki, first discovered living inside freshwater snails, were able to sequence the organism’s genome and found that the amoeba’s genetic code incorporated many genes that are connected to multicellular functions in animals, effectively hinting that the “jump” needed to move from uni- to multicellular may have been less wide and dramatic than previously assumed.
“The finding that many genes involved in these regulatory processes have a premetazoan origin raises the intriguing possibility that the mechanisms required for spatially regulated cell differentiation evolved prior to the appearance of animals,” say the study’s authors.
Inaki Ruiz-Trillo, evolutionary biologist at the Institute of Evolutionary Biology in Barcelona, Spain, and co-author of the study, states, “We show that these early organisms already had some behaviors that we once thought were only in multicellular animals. From there, it would have been a simpler evolutionary leap.”
The endless variations between different types of cells, their functioning and regulatory parts, is what keeps evolutionary biologists guessing at how complex life forms were created.
Recently, scientists discovered a form of eukaryote (organisms with nuclei in their cells) that doesn’t have mitochondria, something speculated at but not thought possible, since mitochondria serve as the power plants in cells. Biologist Anna Karnkowska, now at the University of British Columbia in Vancouver, located the microbe named Monoceromonoides and found no signs of mitochondrial DNA nor any evidence of the kinds of proteins needed for mitochondrial functioning – both pointing to the conclusion that this eukaryote is the one that breaks the mould. “The definition of eukaryotic cells is that they have mitochondria,” says Karnkowska, “[With this study] we overturn this definition.”