It Was Simple Before It Got Complicated

 

These guys, combined, left us with a pretty simple explanation of what the stuff in the Universe is made out of that remains perfectly adequate for a great many purposes, when combined with Newtonian gravity and mechanic's and Maxwell's equations of electromagnetism.

Almost immediately afterwards, however, we discovered quantum mechanics, neutrinos, quarks, gluons, weak force bosons, muons, tau leptons, the Higgs boson, photons, the strong force, the weak force, Special Relativity, and General Relativity, dark matter phenomena, and dark energy phenomena which made everything complicated again. 

The timing doesn't line up perfectly:

The discovery of the neutron and its properties was central to the extraordinary developments in atomic physics in the first half of the 20th century. Early in the century, Ernest Rutherford developed a crude model of the atom, based on the gold foil experiment of Hans Geiger and Ernest Marsden. In this model, atoms had their mass and positive electric charge concentrated in a very small nucleus. By 1920, isotopes of chemical elements had been discovered, the atomic masses had been determined to be (approximately) integer multiples of the mass of the hydrogen atom, and the atomic number had been identified as the charge on the nucleus. Throughout the 1920s, the nucleus was viewed as composed of combinations of protons and electrons, the two elementary particles known at the time, but that model presented several experimental and theoretical contradictions.

The essential nature of the atomic nucleus was established with the discovery of the neutron by James Chadwick in 1932 and the determination that it was a new elementary particle, distinct from the proton.

The uncharged neutron was immediately exploited as a new means to probe nuclear structure, leading to such discoveries as the creation of new radioactive elements by neutron irradiation (1934) and the fission of uranium atoms by neutrons (1938). The discovery of fission led to the creation of both nuclear power and nuclear weapons by the end of World War II. Both the proton and the neutron were presumed to be elementary particles until the 1960s, when they were determined to be composite particles built from quarks.

Newtonian mechanics and gravity (and calculus) all date to the late 1600s. 

Maxwell's equations for electromagnetism were published by 1862. Radioactivity, that would later be explained by the strong force and the weak force, had been discovered in the late 1800s.

Special Relativity (1905) (which is implicitly a part of Maxwell's equations), General Relativity (1915), and rudimentary quantum mechanics (reasonably well developed by the mid-1920s) were already in place before the neutron was discovered. 

Muons were discovered in 1936 although their place in the overall picture wasn't well understood at the time. 

Neutrinos were proposed in 1930, supported by evidence from beta decay in 1934, indirectly observed with physical evidence in 1938, and were first directly observed in 1956. 

The tau lepton was suspected in 1960 but wasn't confirmed until experiments done from 1974-1977.