LITMIR.BIZ

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      However life started, for it to become more complex, cells were required. (A cell is a membrane-bound entity containing molecules to sustain life.) The first cell is thought to have consisted of a membrane composed of phospholipids surrounding self-replicating RNA. Phospholipids are fatty acids consisting of two hydrophobic (water-fearing) tails and a hydrophilic (water-loving) head that function as the building blocks of all biological membranes. When placed in water, phospholipids naturally aggregate with their heads facing out and tails facing in, forming a two-layer barrier. A cell membrane composed of phospholipids functions like the “skin” of a cell, separating its contents from what’s outside it. In early cells, the membrane functioned as an enclosure for the RNA and other molecules, enabling them to operate as a unit with the capacity to reproduce and evolve.

      The first true single-cell organisms to enter the picture were the prokaryotes — bacteria and archaea. The distinguishing characteristic of prokaryotes is that they lack a membrane-bound nucleus (control center), mitochondria (power plant), or other membrane-bound organelles (organized structures within a cell). Early prokaryotes are thought to have been chemoautotrophs — creating their own energy by oxidizing inorganic compounds. Later, approximately 3.5 billion years ago, cyanobacteria evolved, deriving their energy from photosynthesis — using sunlight to synthesize foods from carbon dioxide and water.

Even later, about 1.8 billion years ago, more complex single-cell organisms called eukaryotes appeared. A eukaryote is any organism consisting of one or more cells containing membrane-bound organelles including a distinct nucleus that contains DNA in the form of chromosomes. (You still with us?) Eukaryotes include all living organisms except prokaryotes. In other words, you’re a eukaryote and probably didn’t even know it. However, you do have numerous prokaryotes living on and inside you, most of which are beneficial, and some of which perform essential functions (such as aiding in digestion and synthesizing vitamins your body needs but doesn’t get in its diet or produce on its own).

      And now for a word about metabolism

      Before moving on to the evolution of more complex organisms, we’d like to give a shout out to energy — the power that sustains life and drives evolution. So where does all this energy come from? It comes from a set of life-sustaining chemical reactions in organisms collectively referred to as metabolism. These chemical reactions can be divided into two types:

       Anabolic processes build molecules. When you’re pumping iron at the gym, anabolic processes are at work synthesizing protein molecules to build muscle. Energy is required to fuel anabolic processes.

       Catabolic processes break down molecules into smaller units, often releasing energy; for example, your body can break down sugar or fat molecules to give you the energy to pump that iron.

      All living things use the stuff around them to obtain the energy and molecules they need to carry out vital cellular processes, to reproduce, and, in some cases, to move around. However, every known ecosystem on Earth is fueled by organisms that rely on one of the following two metabolic mechanisms:

       Photosynthesis is the best known of these processes and uses energy from the sun to convert carbon dioxide and water into chemical energy and organic molecules needed for growth. Lucky for us, oxygen is released as a waste product

       Chemosynthesis is less well known and uses energy stored in the chemical bonds of inorganic chemicals such as hydrogen sulfide and methane to make glucose from carbon dioxide and water. Chemosynthesis is what enables bacteria to live near hydrothermal vents at the bottom of the deep blue sea. (See Chapter 5 for more about life that exists around hydrothermal vents.)

Organisms that rely on chemosynthesis and photosynthesis anchor the food webs that enable ecosystems to develop. They’re sort of like the farmers who grow all the food, except they are the food!

      All together now: Multicellular organisms

      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.

      Taking evolution to the next level in the Paleozoic era

      Buckle up, folks … this is where things start to get really interesting. Don’t be turned off by the hard-to-pronounce words or events that can be a little confusing (we had to study this many times before it sunk in). This section begins the story of the evolution of complex life-forms on Earth, and it is a fascinating one. If anything, it reminds us of the incredible dynamism and complex wonder that resulted in … well … us. So read on and marvel at just how amazing this world really is.

      The Paleozoic era spans from about 541 to 251 million years ago, when life underwent enormous diversification. (Paleozoic roughly translates to “ancient life.”) It began with the Cambrian explosion (when nearly all major animal phyla appeared) and ended with The Great Dying (a mass extinction) and can be subdivided into the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian periods.

      The Cambrian period

      The Cambrian period (from 541 to 485.4 million years ago) was a time of massive diversification of life in the ocean. It was thought to have begun as a result of changing ocean chemistry (due to erosion and minerals washing into the ocean) and a boom in oxygen levels due to growing populations of phytoplankton (see Chapters 7 and 8 for more about phytoplankton). During this

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