20.8: Impact on animal evolution
- Page ID
- 22765
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)One of the most intriguing coincidences in the geological record is that “immediately” after the Snowball Earth glaciations, some sedimentary layers hold the very first evidence of multicellular animals preserved as fossil remains. These are the Ediacara, namesakes of the Ediacaran period.
Exactly what the Ediacara were isn’t entirely clear. Animals, probably. What we know is this:
- They were big enough to see with the naked eye, ranging between a couple of centimeters and a couple of meters in length.
- They had morphologically distinct body parts, suggesting that they had differentiated tissues, if not organs.
- They lacked hard parts such as bones, shells, horns or teeth. (The fossils are preserved as impressions of their soft bodies in shale and sandstone.)
- Many appeared to be sessile (rooted in one spot), though others were probably mobile, on the basis of track-like trace fossils associated with some of the body fossils. Some resemble jellyfish, and so may have floated in a similar fashion.
- Most showed bilateral symmetry, though there are others with apparent radial symmetry, and at least one has apparent trilateral (3-part!) symmetry.
In Newfoundland, the end of the Gaskiers glaciation occurred at 579, and the oldest Ediacara fossils are about 5 million years younger. There are sparse examples for several million years, and then at Mistaken Point, a legendary Lagerstätte on the southeastern Avalon Peninsula, a 565 Ma ash bed smothered a huge number of Ediacara on a single bedding plane. A few meters above that, the same thing happened again. This unusual circumstance provides for exceptionally precise time constraints on the age of the organisms killed by the ash fall. Mistaken Point is the oldest known fossil assemblage of diverse, large, biologically complex fossilized animals. All told, Ediacara lived for 33 million years prior to the Cambrian “explosion.” The fact that Ediacara evolved so soon (geologically speaking) after the end of the Snowball Earth suggests the great global catastrophe may somehow have nurtured the conditions for the evolution of these first animals.
But how? One obvious answer is by wiping out whatever came before, and “leveling the playing field” for up-and-coming animals with their new evolutionary innovations. Another straightforward variable was doubtless the increase in shallow sea habitat as glacial melting caused sea level rise, flooding the continents with shallow seas, perhaps an ideal habitat.
More esoteric explanations hinge on an increase in oxygen levels after the Snowball, and how those oxygen levels then provided sufficient fuel for oxygen-consuming animals to exists for the first time. Or perhaps because collagen is an important binding compound that holds animal bodies together, and collagen requires plenty of oxygen to form, that may be the most important aspect of the rise in oxygen. Exactly why oxygen rose isn’t entirely clear, though.
Obviously, being an animal comes with some big advantages, as large bodies with sensing tissues distributed throughout are able to sense gradients well (hot/cold, salty/fresh, toward a greater concentration of food molecules/away), and their large size (relative to microbes and single-celled protists) also potentially helps buffer against short-term hostile variations in environmental conditions. These are both innovations that natural selection could emphasize through time.
Did I Get It? - Quiz
What is the next big milestone in Earth history after the melting of the last Snowball Earth glaciations?
a. The evolution of the first dinosaurs.
b. The evolution of the first arthropods.
c. The evolution of the first animals.
d. The evolution of the first plants.
e. The evolution of the first humans.
- Answer
-
c. The evolution of the first animals.
Which of the following were structures that Ediacaran organisms had?
a. Shells
b. Differentiated tissues
c. Teeth
d. Eyes
- Answer
-
b. Differentiated tissues