Daily Five life questionsanswers and explanations

Real basis

• When did life on Earth appear? Answer: about 3.8 billion years ago or earlier. The earliest firm evidence is about 3.5 billion years old. Some earlier signs go back to 3.8 billion years ago or even earlier, but they are still disputed.

• What were the mitochondria in our cells originally? Answer: bacteria that were engulfed by a cell. Margulis's endosymbiotic theory: mitochondria have their own DNA, and they divide in a way similar to bacteria.

• How similar are human and chimpanzee DNA sequences? Answer: about 99%. Comparing base by base, about 1.2% differ; if inserted and deleted segments are counted too, the difference is about 4%.

• Do dinosaurs still have descendants alive today? Answer: yes: birds. Birds are a branch of the theropod dinosaurs. Crocodiles are close relatives of dinosaurs, not descendants.

• Where did most of the oxygen in the Great Oxidation Event, about 2.4 billion years ago, come from? Answer: photosynthesis by cyanobacteria. Cyanobacteria invented oxygen-producing photosynthesis. The oxygen first rusted the iron in the sea, forming banded iron formations, and only then began to build up in the atmosphere.

• When did the Cambrian explosion of life happen? Answer: about 540 million years ago. Within roughly 20 million years, most of the animal phyla we know today appeared in the fossil record.

• Which was the worst mass extinction in Earth's history? Answer: the end-Permian extinction. About 252 million years ago, about 96% of marine species disappeared, and the trilobites also went extinct completely in this event.

• What were trilobite eyes made of? Answer: calcite crystals. The compound eyes of trilobites were made of calcite crystals. They are among the earliest known complex eyes and are well preserved in fossils.

• How many hearts does an octopus have? Answer: 3. Two supply blood to the gills and one supplies the rest of the body. Its blood is blue, because oxygen is carried by copper-containing hemocyanin.

• Which of these environments can tardigrades (water bears) survive? Answer: the vacuum of space. In 2007, dehydrated tardigrades were exposed to the vacuum of space for ten days, and after they returned to Earth some revived and laid eggs.

• How many kinds of bases does DNA use to encode genetic information? Answer: 4. A, T, G and C. Every three bases encode one amino acid, and proteins are made of 20 amino acids.

• Which animal can make its body “grow backward” to a juvenile state? Answer: the Turritopsis dohrnii jellyfish. When injured or old, Turritopsis dohrnii can turn back into a polyp. It is often called the “immortal jellyfish”, but it can still be eaten or die of disease.

• Which of these is an example of “convergent evolution”? Answer: birds, bats and pterosaurs each evolving wings. Unrelated species facing similar environments independently evolved similar structures.

• By biomass, which group of organisms is the most abundant on Earth? Answer: plants. A 2018 estimate: plants make up about 82% of global biomass, bacteria about 13%, and animals only about 0.4%.

• Which of these can corvids (crow-family birds) do? Answer: New Caledonian crows can make hooked tools from twigs. New Caledonian crows trim twigs into hooks to dig out insects; in experiments, ravens prepare tools for use in the future.

• About how many neurons does the human brain have? Answer: about 86 billion. About 86 billion, and each neuron is connected to several thousand other neurons on average.

• What does the “Red Queen hypothesis” say? Answer: species must keep evolving just to hold their place. The name comes from what the Red Queen says in Through the Looking-Glass: it takes all the running you can do to keep in the same place.

Related research

These studies relate to this topic but are not direct proof of this page.

• The three-domain system: The tree of life was redivided into three domains: Bacteria, Archaea and Eukaryotes.

  Source: Woese, Kandler & Wheelis, PNAS 1990, "Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya"

• Endosymbiotic theory: Margulis proposed that mitochondria and chloroplasts were once independent bacteria that were swallowed and became part of the cell. It was rejected by over a dozen journals.

  Source: Sagan (Margulis), Journal of Theoretical Biology 1967, "On the origin of mitosing cells"

• The double helix: A one-page paper proposed that DNA is a double helix, and base pairing hinted at how genetic information is copied.

  Source: Watson & Crick, Nature 1953, "Molecular structure of nucleic acids: A structure for deoxyribose nucleic acid"

• Natural selection: Darwin and Wallace jointly published the theory of natural selection. The next year, On the Origin of Species came out.

  Source: Darwin & Wallace, Journal of the Proceedings of the Linnean Society 1858, "On the tendency of species to form varieties; and on the perpetuation of varieties and species by natural means of selection"

• The worst mass extinction: Precise dating shows the end-Permian extinction may have lasted only about 60,000 years, matching Siberian volcanic activity.

  Source: Burgess, Bowring & Shen, PNAS 2014, "High-precision timeline for Earth's most severe extinction"