Читать книгу Oceans For Dummies - Joseph Kraynak - Страница 49

Over time, cells began to gather and hang out together, probably not out of loneliness but because sticking together was advantageous to each cell in the group. It’s sort of like schools of fish forming to ward off predators or the way some plants and animals form symbiotic relationships; for example, a sea anemone’s tentacles protect a clownfish from predators while the clownfish chases away butterfly fish that would eat the anemone. Of course, symbiosis is different at the cellular level, but the concept is the same.

For whatever reason, cells began to aggregate forming filaments or mats consisting of the same cell types (colonies) or different cell types (symbiosis). Over time, cells formed clumps and then the clumps formed more and more intricate structures with different parts of each structure performing a distinct function; for example, cells at one end of the structure could be in charge of consuming nutrients, while cells at the other end could be in charge of eliminating waste products.

How the first multicellular organisms developed and then how more complex organisms developed with distinct organs and limbs are topics of speculation. What we do know is that the first multicellular organisms arrived on stage — about 600 million years ago, which is relatively recent in the ocean’s 3.8 billion-year history.

Multicellular life really took off in the Ediacaran period (from 635 to 541 million years ago) with simple organisms such as branching rangeomorphs (animals shaped like leaves), the kimberella (sort of like a slug), and the spriggina (similar to a trilobite; see Figure 3-1), as well as early sponges and cnidarians (jellyfish, anemones, and so on), and soft-bodied organisms that looked like worms, corals, sea-pens, seaweed, and lichen. Dickinsonia is another famous organism from this period, and may be the first animal to move on its own, scampering across the seafloor some 567 to 550 million years ago. Some of these may be the first metazoans — animals with specialized cells and different body sections for different roles.

Photo by James St. John. Licensed under CC BY 4.0.

FIGURE 3-1: Trilobite fossil.