As this book progresses, we will take note of the key evolutionary events that led to our kingdom, then phylum, and so on until we arrive at our species. In fact, cladistic analysis adds several sub-layers to the Linnaean scheme. See the special human ancestor image gallery at the end of the chapter (and the next few chapters) to for illustrations of notable evolutionary phases.
A. Multicellularity and Animals
The most fundamental traits take longest to evolve. After waiting two billion years for eukaryotic cells to appear, we have to wait another billion years to see them come together into multi-cellular plants and animals. The first step was the formation of cell colonies. As cells reproduce, they tend to cluster together to form biofilms like algae or slime mold. It is even common for several different types of cell to live within the same biofilm. Living in numbers can help a colony survive, especially when the cells reproduce sexually. Aside from sex, though, each cell within a colony performs its own vital life functions like eating and breathing. The gradual transition from cell colony to body involved three related breakthroughs: connectivity, communication, and specialization.
The most obvious breakthrough was cell connectivity. A body is not just a bunch of cells. It’s a bunch of cells held together. Cells can attach to each other with junctions in their cell membranes. They can also secrete proteins or minerals to form an extracellular matrix, which can be anything from a soft tissue to a shell that holds the cells together in shape.
Cells within a colony evolved techniques for communicating with each other. Intercellular communication can involve contact or chemicals. Cells can signal their neighbors to activate or deactivate genes, make proteins, divide, stop dividing, or even to die. As an embryo grows, the shape of its body is determined by allowing cells to live in the right places and to die when they are out of place.
To maximize the benefits of colonial life, cells within a colony evolved distinct specialties. The first and most basic form of cell specialization was between those that reproduce sexually and those that reproduce asexually. Cells that divided asexually remained the majority and became body cells. The cells that underwent meiosis became germ cells, which developed in distinct places within the colony / body.
By 600 – 800 million years ago, cell colonies had achieved the requirements for being considered true multicellular organisms: fungi, animals, and plants. Our earliest animal ancestor was the lowly sponge. For the record, there are a suite of traits that define the animal kingdom distinctly from plants or fungi. Only animal cells lack cell walls and produce a vital protein called collagen in the extracellular matrix. Animals ingest other living things and move during some or all of their life cycle (yes, even sponges swim freely when they are larvae!)
Sponges are organized at the cellular level. If you cut out a tiny sample of a sponge, you will find about six or eight types of cell interlaced with each other. That same pattern is repeated throughout the sponge’s entire body. Essentially, the tiny sample is a miniaturized version of the whole organism. You know that is not true of a person. If someone extracted a sample from your finger, it would not look like a small copy of yourself. Animals have evolved large-scale structure. This includes a proliferation of cell types; humans have about 200. More significantly, those cells are now organized into larger units called tissues.
There are four major classes of animal tissue. Muscles are made of muscle tissue, and nerves are made of nervous tissue. Nerves transfer information from one part of the body to another. Individual nerve cells, neurons, relay information in the form of electrical signals. Those signals have a two-way interaction with chemicals called neurotransmitters and hormones within the nervous tissue. Epithelial tissue forms surfaces such as the outer skin and the lining of the gut. Epithelial tissue can fold or branch to form glands. Connective tissue includes the collagen extracellular matrix that holds the body together. It is a framework that supports the epithelial layers. Muscles and nerves are embedded within the connective tissue. 2
These four classes of tissue all evolved in the next wave of animals, metazoa. Some basal metazoans were corals, sea anemones, and jellyfish. Structurally, they had a mouth but an incomplete digestive tract. Food and oxygen were essentially absorbed directly into the cells, and waste was expelled through the mouth!
The trend of cell specialization continued, allowing for increased complexity and efficiency. Early deuterostomes (like sea cucumbers) possessed body cavities and a through gut, with a mouth on one end and an anus on the other. This provided obvious advantages making it easier to eat! Deuterostomes also developed the first true organs. An organ is a specialized combination of different tissues, which work together to perform a specific bodily function. For example, sea cucumbers have a set of tube feet for movement. Blood and vessels evolved at this stage, providing a much more reliable way to carry food and oxygen to all cells. Some portions of the blood vessels were lined with muscle tissue to enable circulation of blood; this was a form of proto-heart. The gut started to form specialized passageways such as the esophagus, stomach, and intestine.
The brain is a swelling in the spinal cord where many nerves are required. The brain and spinal cord form the central nervous system (CNS), where signals from various parts of the body converge and interconnect. Response signals originate in the CNS and travel back to the muscles and organs. Primitive neural circuits operate on auto-pilot, controlling involuntary actions like heartbeat and reflexes.
By 500 – 600 MYA, animals had speciated into 20 or 30 body plans, or phyla. Body plans are defined by symmetry, layers of epithelial tissue, body cavities, and unique body parts. Chordata is the phylum of most interest to us, because we are chordates.
Our symmetry is bilateral, meaning that chordate bodies are comprised of two mirror images, the left and right halves. The mouth end is differentiated from the anus end, and the belly is differentiated from the back. Chordates have three layers of epithelial tissue. The innermost layer forms the gut, the single tube from mouth to anus. The outermost layer is the skin. Between them, the middle epithelial layer forms internal body cavities, which support organs.
Chordates are also distinguished by five particular body parts, which might strike you as random and surprising. We have a hollow spinal cord down the back. The spinal cord is accompanied by a notochord made of cartilage, a semi-rigid connective tissue. Chordates all share an endostyle, effectively the thyroid gland. Chordates are the only animals with a tail that extends past the anus. Finally, all chordates have a series of pharyngeal arches, cartilaginous segments around the throat such as a fish’s gills. “What?!” you protest. “I don’t have a tail or a segmented throat!” Not anymore, but you did before you were born! Even humans have all five chordate features as embryos, certainly proving that we still have the genes for them. In fact, the pharyngeal arches develop into distinct parts of head anatomy during gestation.
Most chordates are vertebrates, with a notochord that has evolved into segmented vertebrae. The earliest vertebrates were jawless fish. Cartilage and scales hardened into bones and teeth. Vertebrates also evolved internal organs such as the liver, kidneys, and pancreas. They were not the first animals with eyes or brains, but they took these organs to new levels. New regions of the brain integrated the outer senses (sight, sound, smell, touch, and taste) and the inner body senses (hunger, pain, balance, etc) into a state of consciousness. The vertebrate brain also generated emotions, which mediate the animal’s reactions to the world around it. This phase of evolution was influenced by the Cambrian explosion, which will be discussed below.
Evolution has an important recurring theme that has not yet been discussed. This is as good a time as any to bring it up. The theme is repurposing. It is common for features that already exist to be modified or exploited in new ways, especially in new environments. The evolution of our ancestors from sea to land involved some successful repurposing of body parts.
Recall that all chordates have pharyngeal arches around the throat, at least during embryonic development. Originally, these pharyngeal arches developed into gills. Some gill arches have become repurposed by evolution as they have assumed useful new shapes. The “first” gill arch (closest to the head) of jawless fish assumed a boomerang-shaped profile and migrated forward in the head. It became affixed to the skull and has forevermore served as the jaw. Arches further back in the body started to bud outward from the side of the animal, becoming paired fins. Fins provide stability. They enable fish to get up off the sea floor and become good swimmers.
Our ancestors were jawed fish called placoderms a little over 400 MYA. An interesting recent discovery shows evidence that placoderms had external sex organs and got pregnant – the earliest fossil evidence of internal fertilization! 3 Prior to intercourse, reproduction was accomplished by spawning. Sperms and eggs were released into the water, where they were free to meet for fertilization. Internal fertilization would be vital for subsequent land animals, as spawning does not work on dry land.
In many lines of fish, cartilage evolved into bone. Fins developed bony support structures separately from the vertebrae. For certain bottom-dwelling lobe-finned fish, a pair of front limbs and a pair of rear limbs proved useful for scuffling along the bottom of the riverbank. 4 The fin bones evolved to become adjoined to the rest of the skeleton. The bone structure of lobed fins started to resemble hands and feet, earning these animals the title of tetrapod (four-legged). Each arm or leg had one upper limb bone, two lower limb bones, a series of small irregular ankle / wrist bones, and then five digits. Tetrapods had bony chest and hip girdles. The skull was separate from the chest, giving tetrapods a neck and a head that could turn freely from the body. These skeletal features are still shared by all members of the tetrapod clade: the amphibians, reptiles, birds, and us mammals.
Meanwhile, air bladders that were originally effective for controlling the depth of swimming were repurposed into lungs. With lungs and a complete skeleton that could support their weight, tetrapods were able to walk onto land. The first tetrapods were also still able to breathe water, making them amphibians. They lived a little less than 400 MYA, and were well-adapted for life in slow, shallow rivers near the sea.
With a complete skeleton that could support their weight, tetrapods were able to venture onto shore. The first tetrapods had primitive swim bladder / lungs 5 and were also still able to breathe water, making them amphibians. They lived a little less than 400 MYA, and were well-adapted for life in slow, shallow rivers near the sea.
In many ways, the first land tetrapods had it made in the shade. They had discovered a whole new realm where they could eat without being eaten. These circumstances created a lot of evolutionary pressure to adapt to the dry-land environment. There is some evidence that their water was running low on oxygen anyway. 6 But think of the serious challenges. If you’ve ever seen a mudskipper flopping around, you know that it isn’t graceful on land. A riverbed is practically a zero-gravity environment. On land, skeletons and muscles had to evolve to support the full weight of the body, without the assistance of buoyancy. The lungs had to be repurposed as dedicated air-breathing organs. Acoustics, too, are completely different in air and water. Organs that had evolved for hearing underwater were useless in the air. This problem was eventually solved by the shrinking and migration of an upper jawbone into the middle ear, where it became the stapes, an auditory bone. Various other land-animal body parts such as the eyelids, diaphragm, and tongue evolved during this Paleozoic poolside party. 7
The last great breakthrough of this chapter was the speciation of our clade the amniotes, as first represented by ancestral reptiles about 310 MYA. An amniotic sac encloses and nourishes the embryo, whether in an egg or the mother’s womb. Amniotic eggs are hard-shelled, offering protection against predators, parasites, and dehydration. Reptiles no longer had a larval stage like tadpoles, but were born or hatched as miniature adults. The perfection of internal fertilization and the appearance of hard-shelled eggs signaled animals’ complete emancipation from the water. Reptiles were also better suited to dry land than amphibians because of their tough, scaly skin.
- Placoderm photo by Chip Clark, 1986, Smithsonian Institution Negative # 86-3346, believed to be in the public domain as the creative work of an agency of the US government. See e.g. https://www.flickr.com/photos/publicresourceorg/493857830 (accessed and archived 8/02/20). ↩
- David G. King, “Connective Tissue Study Guide, ” 2015, http://www.siumed.edu/~dking2/intro/ct.htm#components (accessed and saved 8/10/19). ↩
- John A. Long et al., “Copulation in antiarch placoderms and the origin of gnathostome internal fertilization,” Nature Vol. 517, 196-199 (10/19/2014), https://www.nature.com/articles/nature13825 (accessed 8/11/19). Or if you’re feeling brave, you can even view Flinders University’s computer-generated video of this prehistoric copulation at https://www.youtube.com/watch?v=qmUSfHYpxxQ (posted 10/19/2014, accessed 8/11/19). ↩
- Leonard B. Radinsky, The Evolution of Vertebrate Design, University of Chicago Press (Chicago, 1987) p. 56. ↩
- Norifumi Tatsumi et al., “Molecular developmental mechanism in polypterid fish provides insight into the origin of vertebrate lungs”, Scientific Reports 6, article no. 30580 (7/28/2016), https://www.nature.com/articles/srep30580 (accessed and saved 8/11/19). ↩
- Thomas J. Algeo and Algeo, T.J., and Stephen E. Scheckler, “Terrestrial-marine teleconnections in the Devonian: links between the evolution of land plants, weathering processes, and marine anoxic events”, Philosophical Transactions of the Royal Society of London, B Series 353(1365): 113-130 (1/29/1998), https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1692181/ (accessed and saved 8/11/19). ↩
- Radinsky, op. cit. pp. 77 – 85. ↩
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