Red algae is earliest multicelluar organism
The appearance of mitochondria and chloroplasts brought a tremendous amount of energy to living organisms. They no longer needed to be as energy-efficient as prokaryotes (bacteria tend to have fewer genes and frequently discard some less commonly used genes.). Eukaryotes could directly engulf prokaryotes, shifting the competition among organisms from rapid reproduction to mutual predation.
As producers in the ecosystem, algae were the first to face cruel survival pressures. After their massive proliferation, they not only had to cope with a scarcity of resources but also had to deal with predation from creatures like amoebae. They adopted a strategy of grouping together to counter this predicament. Initially, they existed as organisms somewhere between single-celled and multicellular forms. In harsh survival conditions, they clustered together. While the condition improved, they separated to fend for themselves. Eventually, some single-celled algae began secreting gelatinous compounds, such as pectin, which bound their cell walls together to form colonies. This tight connection made cell separation more challenging. Some colonies of algae developed intercellular bridges to share resources and exchange information. Subsequently, these cells gradually specialized into reproductive cells and photosynthetic cells, as the gene transcription and translation were regulated. They also evolved mechanisms to respond to disturbances of the external environment rapidly. As these specialized cells became more interdependent and functionally integrated, they gradually lost the ability to survive independently, and transition from a group of cells to true multicellular organisms.
This evolutionary process might not have been particularly lengthy. A red algae fossil dating back 1.2 billion years has been identified as the earliest multicellular algae, which is only about 400 million years after the emergence of single-celled algae. Compared to the billion years it took for prokaryotes to evolve into eukaryotes, this evolution happened relatively quickly.
Advantages and disadvantages of multicelluar organism
The size of their bodies is much larger than predators, and their sticky gelatinous substance holds the cells firmly together, making them less susceptible to be engulfed. The different biochemical reactions can interfere with each other to reduce efficiency. When differentiated cells share nutrients and perform specific tasks, the organic matter is produced efficiently, much like an assembly line in modern factories. The internal environment of multicellular algae is more stable than the external environment, it is helpful for cell activities.
Nevertheless, multicellular algae also have significant drawbacks. Most cells are more likely to become somatic cells rather than reproductive cells, which means they delegate the responsibility of reproduction to others. Building such a complex system requires more time and energy, so their reproductive rate is not as fast as that of unicellular organisms. While advantageous in stable and favorable environments, they are not as well-adapted as single-celled organisms to harsh conditions. Additionally, not all cells obey commands and perform their designated tasks diligently; some rogue cells focus on absorbing nutrients and reproducing rapidly, leading to the collapse of the group, similar to cancer cells.