It’s generally thought that the Earth was built out of asteroids called ordinary chondrites. Chondrites contain some of the most ancient minerals in the Solar System, and their composition suggests that they made up the majority of the material that collapsed to form our planet.
But recent observations have shown that the composition of the Earth’s mantle doesn’t match that of ordinary chondrites, suggesting that something else is going on. If the Earth was indeed formed from chondrites, that material must have separated into two different reservoirs, one of which we haven't identified.
Specifically, the Earth’s mantle has a lower ratio of neodymium-142 to -144 than ordinary chondrites. That means that at some point during the Earth’s formation, the chondrites that formed the planet must have differentiated into higher- and lower- ratio clumps (just as mud, stirred into a cup of water, differentiates with the densest concentration of mud ending up on the bottom). Since this requires the Earth to have been very hot, it would have happened within the first 20 million to 30 million years of the planet’s initial accretion.
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