IB Biology HL A2.1 Origins of cells questions

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Question 1

Paper 2Easy6 marks

A research group summarized its estimates for the ages of the first living cells and the last universal common ancestor (LUCA) in Figure 1. The estimates are ranges in Ga, where 1 Ga is one billion years before the present. The estimate for the first living cells was based on fossilized evidence in ancient seafloor hydrothermal-vent precipitates; the estimate for LUCA was based on conserved sequences from genomic analysis.

00.20.40.60.811.21.41.61.822.22.42.62.833.23.43.63.84Estimated time before present / GaFirst living cellsLUCA
(a)

Using Figure 1, state the estimated age range for each of the first living cells and LUCA.

[2]
(b)

State the interval in which the two estimates overlap.

[1]
(c)

State what the fossilized evidence in ancient seafloor hydrothermal-vent precipitates indicates.

[1]
(d)

State what conserved sequences from genomic analysis indicate about LUCA.

[1]
(e)

Using Figure 1, state what the age estimates show about the time scale over which life has been evolving.

[1]

Question 2

Multiple choiceMedium1 mark

A study finds the same genetic code and shared genes across bacteria, archaea and eukaryotes. The oldest known fossil evidence of cells is dated to 3.5 billion years ago, while a molecular-clock estimate places LUCA at 3.8 billion years ago. Which interpretation is supported?

Show answer and explanation

Answer: B. A universal genetic code and genes shared across the three domains support common ancestry and LUCA. A molecular clock estimates LUCA’s age, while fossils provide evidence that cells existed by the fossil date, not the exact beginning of life. Earlier lineages could have become extinct through competition with LUCA and its descendants. A: Treats the oldest known fossil as the exact origin of life; fossil evidence does not establish that life began then. C: Reverses the dating approaches: molecular clocks estimate LUCA’s age, while fossils provide evidence of ancient cells. D: Equates LUCA with the first life and treats its estimated age as excluding earlier forms. Remember: LUCA is the latest common ancestor of extant life, not necessarily the first life.

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